Comment 6 for bug 537640

Since it sounds like you switch between various graphics drivers, it sounds like there is a potential here that you had some bits installed from one of the earlier video drivers which is now interfering. Can you please try booting a LiveCD of Lucid beta1 and see if you can reproduce the issue in that "clean" environment?

One possible cause would be if fglrx bits were installed, and this is documented at https://wiki.ubuntu.com/X/Troubleshooting/FglrxInteferesWithRadeonDriver