So I tapped the power button, and was immediately presented with a black screen and login with a blinking cursor. Typing, however, had no effect, so I tried unplugging and re-plugging the Apple Extended Keyboard (with connected Macally Ice Mouse) from the flat Apple 20" monitor. It worked! I came back to my unchanged desktop.
Perhaps the connection had a short, but I don't think so. I'll bet either the USB communications had broken down somehow, or the screen saver went berzerk. In any case, pulling and reconnecting the USB keyboard fixed the problem.

