Vice grips? I figure you've already tried it, but thats the only way I know of. Normally I'd say fuck it, just drill the thing out with a good bit, but that would cause major problems in a head. If the bolt isn't seized up, a pair of vice grips shoudl do the trick. Unless its hardened, most vice grips will actually be able to compress the head of the head bolt where you squeeze it, giving you a little bit nicer of a surface to try and twist with. If you fail with them, the bolt is hosed, though.