Rusted nuts can be a real pain. If you cant get then to move, a good way I have found is a strong pair of locking grips (mole / vice grips) and plenty of hot water. Pour the hot water slowly over the nut (if possible); this will causethe metal to expand and may slacken it enough for you to get it off.
Obviously this method is less useful for nuts that are underneath something so you cant pour water on it, but there are other ways of warming up the metal......