Whilst testing and update from 5.3 to 5.4 I updated gdm to 
gdm-1:2.16.0-56.el5.x86_64.rpm
Restarting gdm before seeing the login screen a box appears with the 
message:
 	There was an error loading the theme RHEL
 	Cannot open file /usr/share/gdm/themes/RHEL/RHEL.xml

# rpmqv -f /usr/share/gdm/themes/RHEL/*
redhat-logos-4.9.16-1.SL.6.noarch.rpm
...

It seems that /usr/share/gdm/defaults.conf has changed the default theme
# diff /usr/share/gdm/defaults.conf*
459c459
< GraphicalTheme=RHEL
---
> GraphicalTheme=EaseOfBlue

Reversing this change fixes the problem.

-- 
Dr. Andrew C. Aitchison		Computer Officer, DPMMS, Cambridge
[log in to unmask]	http://www.dpmms.cam.ac.uk/~werdna