Binary package hint: xorg
I walked into the room to see the starwars screensaver running, or rather, not running. The display was frozen, and keyboard and mouse input was completely ignored. I ssh'd into my box, and starting killing processes. top showed that the process Xorg was taking nearly 100% of the cpu. Eventually I killed (-9) Xorg. After that I had a zombie [Xorg] process, still taking 100% of the cpu. Eventually, after killing nearly every process on the box, I issued sudo telinit 1. Of course, my ssh connection was severed some time after that. The display remained frozen, and after my ssh connection was severed, any keyboard input produced beeps from the PC speaker. Even pressing the case power button produced a beep! Eventually I did a hard power cycle. Everything seems fine now.
I am not running Xgl. I have a Radeon 7000, and use the "radeon" driver. I had firefox, thunderbird, texmaker, gaim, rhythmbox, gnome-terminal, and panel applets running at the time.
This has happened 3 more times since then. Twice I was able to recover using Alr+Sysrq+k, the third I had to hard power cycle again. Twice it was on a black screen, once on the starwars screensaver again.
I'm going to disable the starwars screensaver and see if that is a decent workaround.