This was left by Emilio as a comment to the Quick Sleep entry from before, but I'm afraid not enough people read the comments so I'm promoting it to full-blown entry.
The keychord CTRL-SHIFT-EJECT instantly blanks the screen, without invoking the screen saver. It's just instant blank. Note that it will not lock the screen if you have passwords turned on for the screen saver, it just makes the screen blank. Still, pretty cool.