Just another data point...
Flashing the motherboard (Gigabyte K8VNXP) bios on my Althon64 system the NVidia stability problems disappeared, so at last I now have a rock solid Athlon64 system
The thing I did was move the Gefore 6800GT over from my Althon system (with the hang on exit problem) to Althon 64 system, and the hang on exit problem disappeared. Both machines have the the 6111 drivers.
The Athlon 64 system has Suse9.1 installed and the Althon system has Suse9.0 installed, so perhaps this is another source for when one hangs and one doesn't.
Other threads which discuss hang on exit also talk about flashiing the bios on the graphics card to fix the crashes.