If the hex head itself is just stripped, use some vise grips clamping straight on the bolt but not going all the way back to the frame so you won't mar the paint. You could always put a little masking tape around the bolt head if you're worried about scratches. If it hasn't been out ever or in awhile tap the bolt lightly a few times with the backside of your pliers as the vibrations will loosen the bolt. Then get yourself a new one of the appropriate diameter, thread pitch and length at your local hardware store, though if you install a pump, chances are the mount will have new bolts. Good luck.
Thanks. Problem is, it's not the head, it's the threads. The screw turns around in the frame, just doesn't come out. I'm not in a huge hurry, so haven't grabbed ahold and pulled with pliers yet - I don't want to screw up the frame if it's the screw that's hosed rather than the frame threads.
I have the exact same problem on my Kona Jake. I took the bottle cage off for a race and screwed the bolts back in. after the race, one bolt came right out...the other is stuck. just spinning in its little hole like an IDIOT.