Emulation, possible extensibility? (Score 2, Interesting)

Nintendo will release eight games for starters, all accurately emulating the NES games...

If they're actually using their own emulator to run the old ROM data for these old games, I wonder how hard it would be (if at all possible) to 'modify' one of the game cartridges to run other NES ROMs.

I dont own a GB, but I'd consider owning one if I could relive my addiction of the original Metroid.