Also in addition to Steve Goddard's post, before the like of Gdk-Warning: gnome-session: Fatal IO error 11, there was this line in my log.
kernel: [21828.132603] Xorg[12291]: segfault at b4fca00
0 ip b4928e66 sp bffa8840 error 7 in nvidia_drv.so[b4850000+706000]
Also in addition to Steve Goddard's post, before the like of Gdk-Warning: gnome-session: Fatal IO error 11, there was this line in my log. drv.so[ b4850000+ 706000]
kernel: [21828.132603] Xorg[12291]: segfault at b4fca00
0 ip b4928e66 sp bffa8840 error 7 in nvidia_
Hopefully that can narrow the problem down a bit.