I've two different systems:
- a Dell Precision T5400 Workstation with 8GB RAM, dual Xeon Quad-Core and a nVidia Quadro FX 550 (256MB VRAM)
- a Dual Opteron 8-core (6128), 32GB RAM and a AMD Radeon 6870 (1GB VRAM)
On the two systems, I have installed openSuSE 12.1 with the proprietary video drivers for their respective graphics cards.

Sometimes suddenly (more often on the Dell workstation), the Xorg process get stucks and goes up to 100% CPU utilization (on a single core, it don't spread over all the cores), and the system become unresponsive.
I can connect through ssh, but I've no chances to stop or kill the stucked Xorg process.

The Magic SysRQ keys are still responsive despite the keyboard seems dead (leds are also unresponsive) and I can force some sync then a poweroff of the system to try to leave the filesystem more consistent than a brutal reset or hard poweroff.

Someone has experienced similar behavior on openSuSE or different distribution?

I've not finded any bug report. I've experienced this issue also in openSuSE 11.4 (I've upgraded to 12.1 only 2 weeks ago).