Here's the end of a verbosity 9 backup. Seems like it's a real error:

Thu Dec  6 10:51:20 2007  Processing changed file var/spool/mail/nfsnobody
Thu Dec  6 10:51:20 2007  Regular copying ('var', 'spool', 'mail', 'nfsnobody')
to /space/backup/rdiff-backup/neuromancer.f8/var/spool/mail/rdiff-backup.tmp.173117
Thu Dec  6 10:51:20 2007  Writing file object to
/space/backup/rdiff-backup/neuromancer.f8/var/spool/mail/rdiff-backup.tmp.173117
Thu Dec  6 10:51:20 2007  Copying attributes from ('var', 'spool', 'mail',
'nfsnobody') to
/space/backup/rdiff-backup/neuromancer.f8/var/spool/mail/rdiff-backup.tmp.173117
Traceback (most recent call last):
  File "/usr/bin/rdiff-backup", line 23, in <module>
    rdiff_backup.Main.Main(sys.argv[1:])
  File "/usr/lib64/python2.5/site-packages/rdiff_backup/Main.py", line 285, in Main
    take_action(rps)
  File "/usr/lib64/python2.5/site-packages/rdiff_backup/Main.py", line 255, in
take_action
    elif action == "backup": Backup(rps[0], rps[1])
  File "/usr/lib64/python2.5/site-packages/rdiff_backup/Main.py", line 308, in
Backup
    backup.Mirror(rpin, rpout)
  File "/usr/lib64/python2.5/site-packages/rdiff_backup/backup.py", line 38, in
Mirror
    DestS.patch(dest_rpath, source_diffiter)
  File "/usr/lib64/python2.5/site-packages/rdiff_backup/backup.py", line 218, in
patch
    ITR(diff.index, diff)
  File "/usr/lib64/python2.5/site-packages/rdiff_backup/rorpiter.py", line 285,
in __call__
    last_branch.fast_process(*args)
  File "/usr/lib64/python2.5/site-packages/rdiff_backup/backup.py", line 490, in
fast_process
    if self.patch_to_temp(rp, diff_rorp, tf):
  File "/usr/lib64/python2.5/site-packages/rdiff_backup/backup.py", line 517, in
patch_to_temp
    rpath.copy_attribs(diff_rorp, new)
  File "/usr/lib64/python2.5/site-packages/rdiff_backup/rpath.py", line 155, in
copy_attribs
    if Globals.change_ownership: rpout.chown(*user_group.map_rpath(rpin))
  File "/usr/lib64/python2.5/site-packages/rdiff_backup/rpath.py", line 790, in
chown
    else: os.chown(self.path, uid, gid)
OverflowError: signed integer is greater than maximum
Exception exceptions.TypeError: "'NoneType' object is not callable" in <bound
method GzipFile.__del__ of <gzip open file
'/space/backup/rdiff-backup/neuromancer.f8/rdiff-backup-data/access_control_lists.2007-12-06T09:24:56-05:00.snapshot.gz',
mode 'wb' at 0x2aaab20c0120 0x2aaab20bcf80>> ignored
Exception exceptions.TypeError: "'NoneType' object is not callable" in <bound
method GzipFile.__del__ of <gzip open file
'/space/backup/rdiff-backup/neuromancer.f8/rdiff-backup-data/extended_attributes.2007-12-06T09:24:56-05:00.snapshot.gz',
mode 'wb' at 0x2aaab20c00a8 0x2aaab20bcef0>> ignored
Exception exceptions.TypeError: "'NoneType' object is not callable" in <bound
method GzipFile.__del__ of <gzip open file
'/space/backup/rdiff-backup/neuromancer.f8/rdiff-backup-data/file_statistics.2007-12-06T09:24:56-05:00.data.gz',
mode 'wb' at 0x2aaab20c0030 0x2aaab20bc248>> ignored
Exception exceptions.TypeError: "'NoneType' object is not callable" in <bound
method GzipFile.__del__ of <gzip open file
'/space/backup/rdiff-backup/neuromancer.f8/rdiff-backup-data/error_log.2007-12-06T09:24:56-05:00.data.gz',
mode 'wb' at 0x2aaab20b8990 0x2aaab20badd0>> ignored
Exception exceptions.TypeError: "'NoneType' object is not callable" in <bound
method GzipFile.__del__ of <gzip open file
'/space/backup/rdiff-backup/neuromancer.f8/rdiff-backup-data/mirror_metadata.2007-12-06T09:24:56-05:00.snapshot.gz',
mode 'wb' at 0x2aaab20b8e40 0x2aaab20bc0e0>> ignored
[root@neuromancer rdiff-backup]# 

Can you provide the output of 'stat var/spool/mail/nfsnobody' ?

I suspect rdiff-backup is making a incorrect assumption about the possible
values of uid/gid. 

raptor> sudo stat /var/spool/mail/nfsnobody
  File: `/var/spool/mail/nfsnobody'
  Size: 0               Blocks: 8          IO Block: 4096   regular empty file
Device: fd00h/64768d    Inode: 1245704     Links: 1
Access: (0660/-rw-rw----)  Uid: (4294967294/nfsnobody)   Gid: (   12/    mail)
Access: 2007-12-07 04:13:21.000000000 -0500
Modify: 2007-11-08 16:42:17.000000000 -0500
Change: 2007-11-08 16:42:17.000000000 -0500
raptor> 

Any hope of getting this fixed? I've got no backups since switching to F8.

Nevermind. Your assumption in #2 seems to be correct. By excluding
/var/spool/mail/nfsnobody, it works fine.

This seems to be a python bug... see: 
http://savannah.nongnu.org/bugs/?17851

for the rdiff-backup bug, and there is apparently a upstream python bug also
about it. I'll try and get some movement upstream on this. 

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This bug will be fixed in 2.5.3 python and later. 

http://svn.python.org/view?rev=61542&view=rev

Unfortunately, that means it's still happening on all stable releases. 
Soon python 2.6.0 will be in rawhide and it should be fixed there.

2.6.0 is in rawhide now. 

Thomas: Can you test this in rawhide? 
Or, failing that, does the exclude workaround work well enough for you now to close this?

The exclude workaround has worked for me for January (comment #5). I'll see if it works without the exclude in rawhide but you can close it anyway.

ok. Will do. 

Please reopen this if you spot additional issues or file a new bug. 

Thanks!