One thing you can try is to get a 6-point socket over it. I have had some success on these using a 1/4 inch socket a little bit smaller than the bleeder is supposed to be, and hammering it on. Relatively cheap Taiwanese sockets that are a little soft work best. The good ones are more likely to crack. You can expect to need a new bleeder after that, and perhaps a new socket as well, but if the bleeder doesn't shear (always a possibility) you can get it out.
I was in Sears the other day and noticed they have a set of special sockets now for getting off damaged nuts and bolts. I didn't get any and don't know how well they work (was in a hurry, so also didn't check sizes), but they looked like a good idea - basically a tapered "hammer-on" socket that's designed to cut into the nut and twist off when you're done. If there's a size that fits bleeders it might be worth trying.
