There is an issue with the log in screen under Mac OS X Lion, where after log out Mac OS wants you to paint the screen with square pixels blocks. It’s quite annoying, but fortunately there is a fix for that.
The issue seems to be caused by the Energy Saver settings and the settings in Security & Privacy.
The solution is to adjust the sleep or the log out options: The display sleep and computer sleep options in Energy Saver must be greater than the time of logout after inactivity in Security & Privacy.
For example if the logout time is set to 9 minutes, the computer and display sleep should be set at least to 10 minutes.