Laptop with fresh Ubuntu 12.04 installation may get white or black screen upon log in. The default purple background appears for a bit, but then entire screen turns white or black while mouse is movable. It may or may not give a message says that need restricted drivers. Mostly, this could be an issue with video driver, log in Unity 2D session and install appropriate driver to fix it.

In log-in screen, select Uinty-2D session and log in. Head to top right corner menu System Settings -> Additional Drivers. Then install the recommended driver.

Will reboot now, and log in Unity 3D to check result.