Hello,
I had the same problem on my wife's laptop after upgrading to Intrepid. The reason is that you have overlapping files from another ATI-drivers. You may fix the problem by removing all xorg video drivers and reinstalling xserver-xorg-video-radeon one. The commands should be something like this (worked for me at least):
* apt-get remove --purge fglrx*
* apt-get remove --purge xserver-xorg-video-ati
* apt-get remove --purge xserver-xorg-video-radeonhd
* apt-get remove --purge xserver-xorg-video-radeon
* apt-get install xserver-xorg-video-radeon
Now run "/etc/init.d/gdm restart" and you'll have your normal graphical login screen. There is no need to modify /etc/X11/xorg.conf
I stole the solution from
https://bugs.launchpad.net/ubuntu/+s...36/comments/15