Just use a dremel tool with one of the thin cutoff wheels to slice into the nut along the axis of the bolt on two sides (or diagonal if the nut is close to the frame). Do this on opposite sides of the nut. Don't cut all the way to the threads, just 90% of the way through. Squeeze the nut with vise grips and it will break into two pieces.
edit: Try the heat thing suggested first as that usually works. I have had some really stuck rounded bolts though that broke apart easily when grooved on two sides with the Dremel cutoff disk.
