Lion: background of the login screen
If you are logging in fresh, it is grey linen.

If you are waking from sleep or display sleep without a screen saver enabled, it is the desktop background.

If you are waking from a screen saver, it is the screen saver itself.

Finally, if you are waking from display sleep or sleep after a screen saver has been ended by display sleep or sleep, it is a black background.

There does not appear to be a way to change this, but keep your eye on apps like Secrets.  If anyone's going to figure it out, it's them.