Has it snapped flush to the frame, ie: is there anything at all to get hold of? If there is, and if your not worried about the paintwork on the frame, you could try heating it up around the seatpost, with a blowtorch, or even the gas burner on your stove. I did that to get a seized post out of an old frame, but I had something to hold on to. Unless anyone has any other ideas, if the length left in the frame isn't that much, you try with a big flathead screwdriver and a big hammer, to hammer it down as far as you can into the frame, and just push a new one down on top of it.
find someone with a welder, weld a flat piece of steel onto it at 90 degrees and use it as a lever. works every time. Unless you have a titianium seatpost. or a carbon one. then you're fucked.