Quite possible - quickest way would just be to touch the two power pins with a piece of metal like the end of a screwdriver; when I'm stripping old cases I keep the reset switch with it's wire and save them for times like this when you need a power button for an ATX board.
LOTS of us in the Distributed Computing forum are running machines naked..er, caseless.
* Look
HERE for a great series of pictures of various "crackracks" dedicated to the Seti@Home project - nearly all are running caseless.
