Hmmm....should have been more clear.
Even if your display is shut, the display still goes back on if you were to unplug an external mouse from the USB port. Plugging it in while the display is shut will also turn the display back on.
I think its because the USB port is always on, and even if you were to plug a mouse into the USB port while the display is shut and the PB/iBook is asleep, plugging it in and having the computer sense it's presence is an input to the computer. This is one of the drawbacks of having your computer detect new hardware immediately. Unplugging your computer is essentially an input because your computer had a connection with a piece of hardware, and when you take it out, it no longer does.
I don't know if that makes any sense, or is clear at all. I'm tired....