(In reply to comment #8)
> I don't have a usable backtrace at hand, but I'm seeing very similar crashes
> in gnome-shell as well with Mesa 9.1 and Linux 3.8.3. With Mesa 9.0.3, I
> haven't noticed any crashes so far. It crashes much more frequently with two
> monitors (both 1920x1200) attached than with just one.

Can you bisect mesa?