If you find X suddenly stopped working on Ubuntu in the last few days you may have the same problem I had. Yesterday a routine update installed xserver-xorg-core 1:1.0.2-0unbuntu10.3. Turns out this "breaks PCI setup for many users", so the solution is to do another update and get xserver-xorg-core 1:1.0.2-0unbuntu10.4 which reverts the change.

Unfortunately I spent several hours trying to fix the problem before doing the sensible thing and checking if an new update was available.

Note to self: always check for an update first.