I just recovered from an apparently-frozen state, without rebooting. The mouse wasn't responding. Keyboard commands had no apparent effect (I tried Command-Tab). Nothing on the screen would move. Command-Option-Escape wouldn't even work. I had just closed an iCal reminder window, and the machine simply froze.
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.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060202161112292