After upgrading to glibc-2.1.2-5, gdm would no longer start
up. Checking the logs, I found that the X server connection
was being rejected. I was going to paste the log message
from /var/gdm/:0.log here, but apparently that's deleted on
a reboot or gdm restart.
Anyway, backdating to RH6 glibc fixed the problem.
Fixed in the latest gdm from rawhide (gdm-1.0.0-39 and up)
I'm looking at the 8/31 rawhide release, and I don't see anything more
recent than gdm-1.0.0-38.