Same problem here. I've got hangs almost daily after log in and starting firefox. Very annoying! Mouse moves still and I can change the terminal and kill X to start again. The "usual suspects" like NvAGP off etc did not dispose the problem.
Asrock Mainboard 939NF4SATA2
GeForce 6100 onboard
BTW, GoogleEarth shakes 3d patterns randomly up, this is an old driver bug, also
When can we expect a driver update?