Comment 1 for bug 160289

This behavior also occurs when the pc is left on and the screensaver kicks in. Attempting to switch users using the fast-user-switch-applet results in a white screen. Pressing Alt+S will reload gdm, however this is probably not the desired solution. A group of users have seen this same issue as seen here:

http://ubuntuforums.org/showthread.php?t=591190

I am using the nvidia-restricted drivers with compiz-fusion on Gutsy.