Ive snaped the center bolt off once but its easy to get out. this would work if you cant get a grip on it with vice grips too. You can get a rheemer (I think its called that) like a reverse drill bit. You just drill a hole in the bolt then jam it in and reverse drilll it out. It should just unscrew out. Drilling the hole weakens the bolt too and makes it come out easily.
:P it jams the cylinder safely so you can take the nut off. also works great for getting the corroded on cylinder head off your car - found that one out yesty