no GDM after preupgrade from F14 -- help!

[Steven Stern]

I ran pre-upgrade this afternoon on my F14 system.  Now, when booting
it goes through the boot process. I can see it start the last thing in
the init list, (Started Display Manager) the screen flashes a few
times, then nothing. If I ALT-F2, I can login and see that gdm is
running, but it's not visible.  Xorg logs show that the radeon driver
is correctly selected.  The main screen is just the list of
"Started...."

Looking for suggestions before I restore from backup or try a clean install.