I use the amd64 architecture, and my VGA card is the RV710 Radeon HD 4550.
For me an fglrx seg fault in firegl_SetSuspectResumeState caused X to not even start up. After updating to Natty, I would get only an 80x25 text login: prompt.
I found the same links mentioned above to expunge fglrx and replace it with the open-source ATI driver. I also had to remove /etc/X11/xorg.conf.
This made it possible for the X login screen to appear, and I could log in using the "Ubuntu Classic (No effects)" option (i.e., Metacity window manager). All attempts to use Compiz, required for Ubuntu Classic or the Unity desktop, fail badly.
I have not been able to solve this problem at all.