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.

This is on an x86-64 system with Radeon HD 4770.