I have the same issue. I think it has to do with the Nvidia 8 series, because I
have a Nvidia 8800 GTX.
I tried the proprietary driver packages, didnt help. I then tried to manually
install the latest driver via Nvidia's website. Again, didnt help.
I have only tested it using XFCE, but I suspect
Public bug reported:
I discovered that mysteriously when shutting down or restarting through
Unity in Oneiric 11.10 with the 3.0.0-17 kernel in the 64-bit version on
my computer resulted in a simple log-out. I could not shut down from the
log in screen. Shutting down via the terminal was just fine
