I've seen a special liquid product for stripped screws at the hardware store. I've never tried it myself but you apply it on the stripped head and after it cures a bit, it's supposed to help you unscrew the thing.
Here's the product that I'm talking about
[http://www.bison.net/US/en/view.do?UID=af993e0fc2c7dd2dc07cf1|ww.bison.net]
If the link doesn't work for you, google it, it's called Bison Grip or Bison Screw Grip.