I vaguely remember having seen something similar before... Here are some ideas to try:
as root to make sure the kmod is installed for the kernel you are using.
Make sure you followed every step in the first post of this thread: http://forums.fedoraforum.org/showthread.php?t=204752
. It might also be worth skimming over the last few pages of that thread.
Check for errors or warnings in /var/log/Xorg.0.log and /var/log/Xorg.0.log.old (grep -e EE -e WW /var/log/Xorg.0.log
Then you could also check whether there are any problems with /etc/X11/xorg.conf, or whether the noveau driver got loaded when it shouldn't (lsmod | grep nouveau