Well, you're screwed (heehee). Unless you can use needle nose pliers to twist it out, you are probably going to have to drill. I would try these first:
Mastercraft Maximum Grab-IT 4-piece Set @
www.canadiantire.ca
Depending on how small your screw is, they're work quite well. If it's too small, you'll have to drill a small hole and use a screw extractor:
http://homerepair.about.com/od/inter..._extractor.htm
I've never had any luck with these things. Your hole has to be dead center, and I always break them anyway.
Another thing you might try. If you have a dremel, cut a slot across the top of the screw and using a slot screwdriver to turn it out.
As a last resort, find out what size the screw is, drill right through with the appropriate size for tapping, and re-tap the hole. Depending on your luck, you may have to tap out to a size bigger than original.
Good luck bud...