This is quick’n'dirty writeup to help others who might have the same problem. I’ll clean this up later in the week …

Ubuntu Jaunty is the new beta version of Ubuntu. It comes with a new version of X.Org and the ATI drivers have been incompatible with this version of X.Org. ATI recently released drivers that support X.Org 1.6. If you have problems with X not starting follow the instructions here:
https://wiki.ubuntu.com/X/Troubleshooting/FglrxInteferesWithRadeonDriver

For me what happened is that X wouldn’t start properly and cause my Lenovo T400 to freeze and lock up. The laptop would boot but when X would start (when the login screen appears) I’d get a black screen with red lines at the top. To solve this I booted to runlevel 1, also known as failsafe mode. If your using a laptop with wireless, I suggest you connect directly to your router using a network cable, otherwise you may have to configure wireless manually (it’s not that hard).

When your system starts, select failsafe mode, on the next screen to appear choose the option to go to a “root shell with networking”.

Next remove the install ATI drivers and install from scratch the ones you need:

sudo apt-get remove –purge xorg-driver-fglrx xserver-xorg-video-ati xserver-xorg-video-radeon
sudo apt-get install xserver-xorg-video-ati
sudo apt-get install –reinstall libgl1-mesa-glx libgl1-mesa-dri
dpkg-reconfigure -phigh xserver-xorg

If this doens’t work for you see the Ubuntu page mentioned above.