The prefdm script depends on the entry in /etc/sysconfig/desktop actually
reflecting
the desktop being run on the system. That is, it presumes that if
/etc/sysconfig/desktop
contains the line DESKTOP="KDE", then KDE--and kdm--is actually installed.
Not always
true. For example, I installed Red Hat 6.2 as a KDE workstation, but later
decided to 1) use startx instead of having a graphical login, and 2)
decided to get rid of KDE in favor of Blackbox, and yet a 
few months more months down the line, I decided to use Helix GNOME. Up to
this point, 
/etc/sysconfig/desktop still was unchanged, containing the line
DESKTOP="KDE", and when I decided 
to use a graphical login again by changing the runlevel back to 5, prefdm
choked, and I didn't get a 
graphical login screen at all--not a good failsafe. Changing the last few
lines of the prefdm script from

	if [ -n "$preferred" ] && which $preferred >/dev/null 2>&1; then 
		exec `which $preferred` $*
	fi

to

	if [ -n "$preferred" ] && which $preferred >/dev/null 2>&1; then 
		exec `which $preferred` $*
	else
		exec `which xdm` $*
	fi

at least gets me *a* graphical login instead of an error message at a
nongraphical login prompt, even
if /etc/sysconfig/desktop is messed up. 

(As a sidenote, changing /etc/sysconfig/desktop so that it read
DESKTOP="GNOME" got me the 
graphical login I wanted, namely gdm, but that's not the point of this
report.)