Ubuntu 12.04 Nvidia GTX 550-Ti x-swat Nvidia driver
Using 2 monitors. When screen locks there is a blue box on top of the wallpaper and password dialog box that hides the field for entering your password or switching users.
Problem is on 2 systems with similar hardware (Nvidia card, x-swat driver, dual monitors)
You can still type your password in blindly and hit enter to login but it's irritating to not be able to see the dialog box.