Can't remember seeing this anywhere, so here goes. After spilling a drink onto my keyboard (by mistake of course), I furiously mopped it up and in doing so, brought up the shutdown dialogue box. Nothing strange there you say ... well this is a PC PS/2 keyboard connected through a Belkin PS/2 / USB 4-port KVM.
I then tracked it down to the Scroll Lock key on the top row above the home key on mine. This may vary from keyboard to keyboard, depending on whether you have one of the flashy multimedia type keyboards with the multiple array of buttons.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040714024517925