Assuming the screw head is fully stripped: First try to use a Dremel or similar tool with a disc-shaped bit to "cut" a new slot into the screw and use a flat screwdriver to get it out. If that doesn't work, use a sphere-shaped bit to drill off the whole screw head (carefully! ).

Slot-cutting: http://www.youtube.com/watch?v=zesGk1OdwJ0#t=2:57

Drilling the whole head off: http://www.flickr.com/photos/thp4/74...57630279222804

Good luck!