When using gnome 3.8, one witnesses crashes of the gnome-shell process which seem to be caused by the inability of the driver to initialize a framebufffer object. Gnome shell mostly crashes at user switching (specifically, at tty switching), but crashes have also been observed at login or logout.
The Gnome developers already looked at the issue and believe it is a NVIDIA driver bug. The bug report and analysis can be followed here:
Please let me know if you need more information. Thank you for your help