WD40 is actually, IMHO, the stuff to use. Keep it off porous parts (ceramic capacitors) or anything you're worried about losing the printing on if you're paranoid. The only downside (as far as I'm concerned) is that it's not that great of a lubricant or penetrant (actually being a Water Displacer), and of course you don't want to get it on moving parts or potentiometers or anything like that.
The worst part is that, given that it is volatile and will mostly just evaporate away in a month, the hardware will reek like WD-40 when warm until it's all gone.
(Yes, I know people differ with me on this, but I'd like to see an example of a PCB 'ruined' by WD-40 that wasn't doomed otherwise... They put these things through soldering ovens, hose them down with all kinds of fluxes/flux-removers, install them under the hood of your car without a problem, and a few petroleum distillates are going to be worse than battery leakage etching through the board? ... or the risk of trying to hold that drill or Dremel steady? ;-))
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Another option is to use a nut-driver instead of a screwdriver, if these are 'computer screws' of the sort that accept one... and that hasn't been tried already. It's amazing how much more leverage one of those things usually gives you, since you can press down a bit and loosen any 'freeze' to the threads without the risk of sending a sharp point or chisel skittering across the board.
Edit: Ah, okay, they're probably not that style. So one more thought -- get a few small Allen wrenches of a size that just fits the stripped-up hole(s) -- at least in the US, they can usually be had singly at a hardware store for less than $1 each, and some fast-set epoxy. Very carefully apply a *small* blob of epoxy to either the allen wrench or the screw, match the two up, and hold it until it can stand on its own, then let it cure for as long as you can stand. Now when you go to twist the Allen wrench, the worst that can happen is the epoxy will snap from the screw head and you'll be no worse than you started. (This does assume the holes are stripped enough to cram in an allen wrench thick enough to not just bend.)
