I recently installed Ubuntu 13.04 and sometimes (not every time, I can't seem to identify a specific cause) the log in screen will be black. That is, I boot up, the terminal "ubuntu login: " shows for a second, it disappears leaving only a single white line in the upper left hand corner of the screen and my mouse cursor appears. At this point, if my speakers are on, it will play the standard sound that plays when you enter the login screen, and if I enter my password and hit enter it will take me to the desktop no problem. But I wouldn't describe this as standard behaviour.
Hopefully someone has encountered this problem and can help.