I'm using Unity on Ubuntu 12.04. My lock screen has always been white. It was also white when I used Gnome 3. If I move the mouse, click, or begin typing my password, my wallpaper appears in the background with the password prompt window, as you'd expect. But I know that the lock screen is not intended to work this way. The Unity Greeter, the Lightdm session manager, is supposed to operate the lock screen.
It's supposed to look like this image here: http://cloudfront.omgubuntu.co.uk/wp-content/uploads/2012/02/new-lightdm-lockscreen-500x351.jpg
How do I fix this? I can't seem to find anyone else with this problem and I don't know what I need to reinstall or change.
The current answer, that it's not a screensaver, is wrong. The white screen is unacceptable and is not Unity's intended behavior.