Comment 123 for bug 546578

I had the same bug. After logging more than one user and logout one of them I got the black screen.

I've updated nvidia drivers to 173.14.27 and the bug has gone.

Adding repo with the latest drivers for nvidia cards

sudo add-apt-repository ppa:ubuntu-x-swat/x-updates
sudo apt-get update
sudo apt-get upgrade

Now I can switch between users and logging out at any moment.

$ nvidia-settings -q NvidiaDriverVersion

  Attribute 'NvidiaDriverVersion' (connie-desktop:0.0): 173.14.27

$ uname -a
Linux connie-desktop 2.6.32-25-generic #43-Ubuntu SMP Wed Sep 1 09:46:39 UTC 2010 i686 GNU/Linux