Not sure how this one is to be classified, but while getting ready to boot my G3 PowerBook into Target Disk mode, I found something interesting. By connecting the PowerBook to the front of the G5 (the G5 is off) with a 6-pin FireWire cable, and then turning on the PowerBook, I was able to access and mount the two chained Firewire drives connected to the back panel of the G5. I guess the front and rear FireWire ports are somehow connected and still drawing power even when the G5 is "off."
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050525233948736