The post-install script has:

chkconfig --add cups || true

and I think it's been there since the first CUPS package we shipped.

How can I reproduce this problem, since I haven't seen it on any upgrade I've
performed here?

Hmm. This has happened to me three times now on the same machine, so I thought
it would be general. If you're saying it might be specific to that machine,
things get more complicated. 

The machine in question used to be an 8.0, was upgraded to 9. Here is what I did
in responce to your question:

[root@kverulanten root]# cp -var /etc/cups /etc/cups.sav

[root@kverulanten root]# rpm -q cups
cups-1.1.17-13.3
[root@kverulanten root]# strace -v -f -o cupsstrace1 -s 256 rpm -Uhv
cups-1.1.17-13.i386.rpm cups-libs-1.1.17-13.i386.rpm
(processes attaching and detaching, long hang)
^C
error: %postun(cups-1.1.17-13.3) scriptlet failed, exit status 0
^C
(no reaction)
^C^C^C
(still no reaction)
(other terminal)
kill -9 [strace PID)
Killed

[root@kverulanten root]# rpm -q cups
cups-1.1.17-13.3
cups-1.1.17-13

Oops, not very good. I thought --force was equivalent to --replacefiles
--replacepackage and the rest of their gang. Better start all over. 

[root@kverulanten root]# rpm -e cups-1.1.17-13.3
[root@kverulanten root]# rpm -e cups-libs-1.1.17-13.3

[root@kverulanten root]# rpm -qa |grep cups
cups-libs-1.1.17-13
cups-1.1.17-13

[root@kverulanten root]# cp -var /etc/cups.sav/* /etc/cups/

[root@kverulanten root]# chkconfig --list cups
service cups supports chkconfig, but is not referenced in any runlevel (run
'chkconfig --add cups')
[root@kverulanten root]# chkconfig --add cups
[root@kverulanten root]# chkconfig --list cups
cups            0:off   1:off   2:on    3:on    4:on    5:on    6:off

OK, I guess now I can do the update. 

[root@kverulanten root]# strace -v -f -o cupsstrace2 -s 256 rpm -Uhv
cups-1.1.17-13.3.i386.rpm cups-libs-1.1.17-13.3.i386.rpm
(preparing, processes detaching and attaching, finishes without error)

[root@kverulanten root]# chkconfig --list cups
service cups supports chkconfig, but is not referenced in any runlevel (run
'chkconfig --add cups')

Right. It did it again. 

[root@kverulanten root]# ls -l cupsstrace*
-rw-r--r--    1 root     root      9223982 Jun  9 16:59 cupsstrace1
-rw-r--r--    1 root     root      9598399 Jun  9 17:11 cupsstrace2

Perhaps it's not a very good idea to create 20 MB of attachments here. Lemme
look. /sbin/chkconfig is called left and right, but I don't really see what goes
wrong. The last invocation of chkconfig is a --del though.

[root@kverulanten root]# grep chkconfig cupsstrace2
4280  pread(4,
"\0\0\0\0\1\0\0\0\320\20\0\0\270\0\0\0\317\20\0\0\1\0\346\17\0\7cy logic to
things that already have\n  start/stop priorities\n- fix silly display bug in
--config\0- chkconfig: LSB support\0- alternatives: handle initscripts too;
--initscript command-line option\n- chkconfig/ntsysv (and serviceconf"..., 4096,
17629184) = 4096
4280  pread(4,
"\0\0\0\0\1\0\0\0\335\7\0\0\334\7\0\0B\6\0\0\1\0\346\17\0\7n/grep\0/bin/sed\0/bin/sh\0/bin/sh\0/bin/sh\0/bin/sh\0/bin/sh\0/etc/redhat-release\0/sbin/arping\0/sbin/chkconfig\0/sbin/fuser\0/sbin/ip\0/sbin/sysctl\0/usr/sbin/groupadd\0SysVinit\0bash\0config(initscripts)\0dev\0e2fsprogs\0fileutils\0gawk\0libc.so.6"...,
4096, 8245248) = 4096
4280  pread(4, "\0\0\0\0\1\0\0\0L\6\0\0K\6\0\0M\6\0\0\1\0\346\17\0\7DOMAIN is
set\0- Make the random seed file mode 600.\0- bring down ppp devices if
ifdown-ppp is called while ifup-ppp is sleeping.\0- moved to new chkconfig
conventions\0- fixed rc.sysinit for hwclock compatibility\0- run \'ulimit -c
0\'"..., 4096, 6602752) = 4096
4280  pread(3,
"\0\0\0\0\1\0\0\0\335\30\0\0\334\30\0\0\336\30\0\0\1\0\346\17\0\7\0\0\0@\0\0\0@\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0\0@\1\0\0J\1\0\0J\1\0\0J\0\0\0@/bin/sh\0/bin/sh\0/bin/sh\0/bin/sh\0/bin/sh\0/etc/rc.d/init.d\0/usr/sbin/alternatives\0chkconfig\0config(LPRng)\0fileutils\0gawk\0libc.so.6\0libc.so.6(GLIBC_2.0)\0libc.so."...,
4096, 26071040) = 4096
4280  pread(3,
"\0\0\0\0\1\0\0\0\335\30\0\0\334\30\0\0\336\30\0\0\1\0\346\17\0\7\0\0\0@\0\0\0@\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0@\0\0\0\0@\1\0\0J\1\0\0J\1\0\0J\0\0\0@/bin/sh\0/bin/sh\0/bin/sh\0/bin/sh\0/bin/sh\0/etc/rc.d/init.d\0/usr/sbin/alternatives\0chkconfig\0config(LPRng)\0fileutils\0gawk\0libc.so.6\0libc.so.6(GLIBC_2.0)\0libc.so."...,
4096, 26071040) = 4096
4280  write(19, "#!/bin/sh\n#\n# \"$Id: cups.sh,v 1.10 2000/03/30 05:19:16 mike
Exp $\"\n#\n#   Startup/shutdown script for the Common UNIX Printing System
(CUPS).\n#\n#   Linux chkconfig stuff:\n#\n#   chkconfig: 2345 90 10\n#  
description: Startup/shutdown script for the Common UN"..., 2312) = 2312
4280  write(18, "/sbin/chkconfig --del cupsd 2>/dev/null || true # Make sure old
versions aren\'t there anymore\n/sbin/chkconfig --add cups ||
true\n/usr/sbin/alternatives --install /usr/bin/lpr print /usr/bin/lpr.cups 40
\\\n\t --slave /usr/bin/lp print-lp /usr/bin/lp.cups \\\n\t "..., 1312) = 1312
4283  read(3, "/sbin/chkconfig --del cupsd 2>/dev/null || true # Make sure old
versions aren\'t ", 80) = 80
4283  read(255, "/sbin/chkconfig --del cupsd 2>/dev/null || true # Make sure old
versions aren\'t there anymore\n/sbin/chkconfig --add cups ||
true\n/usr/sbin/alternatives --install /usr/bin/lpr print /usr/bin/lpr.cups 40
\\\n\t --slave /usr/bin/lp print-lp /usr/bin/lp.cups \\\n\t "..., 1312) = 1312
4284  execve("/sbin/chkconfig", ["/sbin/chkconfig", "--del", "cupsd"],
["HOSTNAME=kverulanten.graenden.org", "SHELL=/bin/bash", "TERM=xterm",
"HISTSIZE=1000", "SSH_CLIENT=192.168.1.1 2899 22", "QTDIR=/usr/lib/qt-3.1",
"SSH_TTY=/dev/pts/0", "USER=root",
"LS_COLORS=no=00:fi=00:di=00;34:ln=00;36:pi=40;33:so=00;35:bd=40;33;01:cd=40;33;01:or=01;05;37;41:mi=01;05;37;41:ex=00;32:*.cmd=00;32:*.exe=00;32:*.com=00;32:*.btm=00;32:*.bat=00;32:*.sh=00;32:*.csh=00;32:*.tar=00;31:*.tgz=00;31:*.arj=00;31:*.taz=00;31:*.lz"...,
"USERNAME=root", "PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin",
"MAIL=/var/spool/mail/root", "_=/sbin/chkconfig", "PWD=/",
"INPUTRC=/etc/inputrc", "LANG=en_US.iso88591",
"SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass", "HOME=/root", "SHLVL=2",
"BASH_ENV=/root/.bashrc", "LOGNAME=root", "SSH_CONNECTION=192.168.1.1 2899
192.168.1.8 22", "LESSOPEN=|/usr/bin/lesspipe.sh %s", "DISPLAY=localhost:10.0",
"G_BROKEN_FILENAMES=1"]) = 0
4284  open("/usr/share/locale/en_US.iso88591/LC_MESSAGES/chkconfig.mo",
O_RDONLY) = -1 ENOENT (No such file or directory)
4284  open("/usr/share/locale/en_US/LC_MESSAGES/chkconfig.mo", O_RDONLY) = -1
ENOENT (No such file or directory)
4284  open("/usr/share/locale/en.iso88591/LC_MESSAGES/chkconfig.mo", O_RDONLY) =
-1 ENOENT (No such file or directory)
4284  open("/usr/share/locale/en/LC_MESSAGES/chkconfig.mo", O_RDONLY) = -1
ENOENT (No such file or directory)
4283  read(255, "/sbin/chkconfig --add cups || true\n/usr/sbin/alternatives
--install /usr/bin/lpr print /usr/bin/lpr.cups 40 \\\n\t --slave /usr/bin/lp
print-lp /usr/bin/lp.cups \\\n\t --slave /usr/bin/lpq print-lpq
/usr/bin/lpq.cups \\\n\t --slave /usr/bin/lprm print-lprm /usr/bin"..., 1312) = 1218
4285  execve("/sbin/chkconfig", ["/sbin/chkconfig", "--add", "cups"],
["HOSTNAME=kverulanten.graenden.org", "SHELL=/bin/bash", "TERM=xterm",
"HISTSIZE=1000", "SSH_CLIENT=192.168.1.1 2899 22", "QTDIR=/usr/lib/qt-3.1",
"SSH_TTY=/dev/pts/0", "USER=root",
"LS_COLORS=no=00:fi=00:di=00;34:ln=00;36:pi=40;33:so=00;35:bd=40;33;01:cd=40;33;01:or=01;05;37;41:mi=01;05;37;41:ex=00;32:*.cmd=00;32:*.exe=00;32:*.com=00;32:*.btm=00;32:*.bat=00;32:*.sh=00;32:*.csh=00;32:*.tar=00;31:*.tgz=00;31:*.arj=00;31:*.taz=00;31:*.lz"...,
"USERNAME=root", "PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin",
"MAIL=/var/spool/mail/root", "PWD=/", "INPUTRC=/etc/inputrc",
"LANG=en_US.iso88591", "SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass",
"HOME=/root", "SHLVL=2", "BASH_ENV=/root/.bashrc", "LOGNAME=root",
"SSH_CONNECTION=192.168.1.1 2899 192.168.1.8 22",
"LESSOPEN=|/usr/bin/lesspipe.sh %s", "DISPLAY=localhost:10.0",
"G_BROKEN_FILENAMES=1", "_=/sbin/chkconfig"]) = 0
4287  execve("/bin/sh", ["sh", "-c", "/sbin/chkconfig --add lpd"],
["HOSTNAME=kverulanten.graenden.org", "SHELL=/bin/bash", "TERM=xterm",
"HISTSIZE=1000", "SSH_CLIENT=192.168.1.1 2899 22", "QTDIR=/usr/lib/qt-3.1",
"SSH_TTY=/dev/pts/0", "USER=root",
"LS_COLORS=no=00:fi=00:di=00;34:ln=00;36:pi=40;33:so=00;35:bd=40;33;01:cd=40;33;01:or=01;05;37;41:mi=01;05;37;41:ex=00;32:*.cmd=00;32:*.exe=00;32:*.com=00;32:*.btm=00;32:*.bat=00;32:*.sh=00;32:*.csh=00;32:*.tar=00;31:*.tgz=00;31:*.arj=00;31:*.taz=00;31:*.lz"...,
"USERNAME=root", "PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin",
"MAIL=/var/spool/mail/root", "PWD=/", "INPUTRC=/etc/inputrc",
"LANG=en_US.iso88591", "SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass",
"HOME=/root", "SHLVL=2", "BASH_ENV=/root/.bashrc", "LOGNAME=root",
"SSH_CONNECTION=192.168.1.1 2899 192.168.1.8 22",
"LESSOPEN=|/usr/bin/lesspipe.sh %s", "DISPLAY=localhost:10.0",
"G_BROKEN_FILENAMES=1", "_=/usr/sbin/alternatives"]) = 0
4287  execve("/sbin/chkconfig", ["/sbin/chkconfig", "--add", "lpd"],
["HOSTNAME=kverulanten.graenden.org", "TERM=xterm", "SHELL=/bin/bash",
"HISTSIZE=1000", "SSH_CLIENT=192.168.1.1 2899 22", "QTDIR=/usr/lib/qt-3.1",
"SSH_TTY=/dev/pts/0", "USER=root",
"LS_COLORS=no=00:fi=00:di=00;34:ln=00;36:pi=40;33:so=00;35:bd=40;33;01:cd=40;33;01:or=01;05;37;41:mi=01;05;37;41:ex=00;32:*.cmd=00;32:*.exe=00;32:*.com=00;32:*.btm=00;32:*.bat=00;32:*.sh=00;32:*.csh=00;32:*.tar=00;31:*.tgz=00;31:*.arj=00;31:*.taz=00;31:*.lz"...,
"USERNAME=root", "MAIL=/var/spool/mail/root",
"PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin", "_=/sbin/chkconfig",
"INPUTRC=/etc/inputrc", "PWD=/", "LANG=en_US.iso88591",
"SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass", "SHLVL=3", "HOME=/root",
"BASH_ENV=/root/.bashrc", "LOGNAME=root", "SSH_CONNECTION=192.168.1.1 2899
192.168.1.8 22", "LESSOPEN=|/usr/bin/lesspipe.sh %s", "DISPLAY=localhost:10.0",
"G_BROKEN_FILENAMES=1"]) = 0
4288  execve("/bin/sh", ["sh", "-c", "/sbin/chkconfig --del cups"],
["HOSTNAME=kverulanten.graenden.org", "SHELL=/bin/bash", "TERM=xterm",
"HISTSIZE=1000", "SSH_CLIENT=192.168.1.1 2899 22", "QTDIR=/usr/lib/qt-3.1",
"SSH_TTY=/dev/pts/0", "USER=root",
"LS_COLORS=no=00:fi=00:di=00;34:ln=00;36:pi=40;33:so=00;35:bd=40;33;01:cd=40;33;01:or=01;05;37;41:mi=01;05;37;41:ex=00;32:*.cmd=00;32:*.exe=00;32:*.com=00;32:*.btm=00;32:*.bat=00;32:*.sh=00;32:*.csh=00;32:*.tar=00;31:*.tgz=00;31:*.arj=00;31:*.taz=00;31:*.lz"...,
"USERNAME=root", "PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin",
"MAIL=/var/spool/mail/root", "PWD=/", "INPUTRC=/etc/inputrc",
"LANG=en_US.iso88591", "SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass",
"HOME=/root", "SHLVL=2", "BASH_ENV=/root/.bashrc", "LOGNAME=root",
"SSH_CONNECTION=192.168.1.1 2899 192.168.1.8 22",
"LESSOPEN=|/usr/bin/lesspipe.sh %s", "DISPLAY=localhost:10.0",
"G_BROKEN_FILENAMES=1", "_=/usr/sbin/alternatives"]) = 0
4288  execve("/sbin/chkconfig", ["/sbin/chkconfig", "--del", "cups"],
["HOSTNAME=kverulanten.graenden.org", "TERM=xterm", "SHELL=/bin/bash",
"HISTSIZE=1000", "SSH_CLIENT=192.168.1.1 2899 22", "QTDIR=/usr/lib/qt-3.1",
"SSH_TTY=/dev/pts/0", "USER=root",
"LS_COLORS=no=00:fi=00:di=00;34:ln=00;36:pi=40;33:so=00;35:bd=40;33;01:cd=40;33;01:or=01;05;37;41:mi=01;05;37;41:ex=00;32:*.cmd=00;32:*.exe=00;32:*.com=00;32:*.btm=00;32:*.bat=00;32:*.sh=00;32:*.csh=00;32:*.tar=00;31:*.tgz=00;31:*.arj=00;31:*.taz=00;31:*.lz"...,
"USERNAME=root", "MAIL=/var/spool/mail/root",
"PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin", "_=/sbin/chkconfig",
"INPUTRC=/etc/inputrc", "PWD=/", "LANG=en_US.iso88591",
"SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass", "SHLVL=3", "HOME=/root",
"BASH_ENV=/root/.bashrc", "LOGNAME=root", "SSH_CONNECTION=192.168.1.1 2899
192.168.1.8 22", "LESSOPEN=|/usr/bin/lesspipe.sh %s", "DISPLAY=localhost:10.0",
"G_BROKEN_FILENAMES=1"]) = 0
4280  write(6, "if [ \"$1\" = \"0\" ]; then\n\t/sbin/service cups stop >
/dev/null 2>&1\n\t/sbin/chkconfig --del cups\n        /usr/sbin/alternatives
--remove print /usr/bin/lpr.cups\nfi\nexit 0", 167) = 167
4292  read(255, "if [ \"$1\" = \"0\" ]; then\n\t/sbin/service cups stop >
/dev/null 2>&1\n\t/sbin/chkconfig --del cups\n        /usr/sbin/alternatives
--remove print /usr/bin/lpr.cups\nfi\nexit 0", 167) = 167
4297  read(255, "#!/bin/sh\n#\n# \"$Id: cups.sh,v 1.10 2000/03/30 05:19:16 mike
Exp $\"\n#\n#   Startup/shutdown script for the Common UNIX Printing System
(CUPS).\n#\n#   Linux chkconfig stuff:\n#\n#   chkconfig: 2345 90 10\n#  
description: Startup/shutdown script for the Common UN"..., 2312) = 2312

I put both strace outputs at http://www.provocation.net/tmp/cupsstrace1 and
http://www.provocation.net/tmp/cupsstrace2 . Please let me know when you've
grabbed them so I can remove them.

chkconfig --del cups only happens when, after the package removal, there are no
instances of cups left.  So on upgrade, this happens:

[original cups package is installed]
new cups package is unpacked onto filesystem
new cups package post scriptlet runs (chkconfig --add cups)
original cups preun scriptlt runs (and does nothing, since $1 == 1)

So it looks to me like your RPM database is confused.  You could force-remove
all cups packages, then install the latest ones, and see if the problem doesn't
go away.

I've had this problem with cups since day 1, so I doubt it's the rpm database.
But that's easy to test:

[root@kverulanten root]# rpm -e cups
warning: /etc/cups/printers.conf saved as /etc/cups/printers.conf.rpmsave
[root@kverulanten root]# rpm -e --nodeps cups-libs
[root@kverulanten root]# rpm --rebuilddb

> You could force-remove all cups packages, then install the latest ones, 
> and see if the problem doesn't go away.

The problem occurs when upgrading, so installing the latest packages would
bypass it, not solve it. Besides, if all I was trying to do was to get my cups
working, a simple "chkconfig --add cups" would be sufficient. I filed a bug
report to help you figure what goes wrong in the upgrade procedure and why, not
to get my cups working. If you're interested, here is what happens:

I have now no cups installed and the rpm database has just been rebuilt. 

[root@kverulanten root]# rpm -hiv cups-1.1.17-13.i386.rpm
cups-libs-1.1.17-13.i386.rpm
Preparing...                ########################################### [100%]
   1:cups-libs              ########################################### [ 50%]
   2:cups                   ########################################### [100%]
[root@kverulanten root]# chkconfig --list cups
service cups supports chkconfig, but is not referenced in any runlevel (run
'chkconfig --add cups')
[root@kverulanten root]# chkconfig --add cups
[root@kverulanten root]# chkconfig --list cups
cups            0:off   1:off   2:on    3:on    4:on    5:on    6:off

Cups was not automatically turned on, which is correct behaviour according to
installation policies; network services must not be turned on by default. Having
turned cups on manually, it's now time for the update.

[root@kverulanten root]# cp -arf /etc/cups.sav /etc/cups
[root@kverulanten root]# service cups start
Starting cups:                                             [  OK  ]
[root@kverulanten root]# rpm -Uhv cups-1.1.17-13.3.i386.rpm
cups-libs-1.1.17-13.3.i386.rpm
Preparing...                ########################################### [100%]
   1:cups-libs              ########################################### [ 50%]
   2:cups                   ########################################### [100%]
[root@kverulanten root]# chkconfig --list cups
service cups supports chkconfig, but is not referenced in any runlevel (run
'chkconfig --add cups')

If you now say that my rpm database is confused, you would be suggesting that
there is a bug in rpm. Personally I don't think anything is wrong with rpm.
Something is wrong with the cups %post and those strace outputs should help you
figure it. Having proven your assumption on the cause of this wrong, I am
re-opening the bug.

It looks like you forgot to run 'redhat-switch-printer' to set CUPS to be the
preferred spooler.