Interesting, had it been inactive for a while when this happened ?
Yes, but dead with flashing lights not dead dead
I've seen xscreensaver lock up Victoria's laptop to the point I had to ssh in and kill it twice now. I've not managed to pin down the exact screensaver thats causing this. I can't even point the finger at DRI as its running unaccelerated 3d.
Switch to the 2D to unaccelerated and repeat the experiment. Good candidates will be accelerated arc support (because only X screensaver uses it) and similar things