You would have to assume that Nintendo is very aware of the whole Flash Cart phenomenon with the GBA.
So you would have to imagine that they have done whatever they can to prevent that on the DS. But the DS does play GBA software.
So the question is, if somebody has a flash cart with Pocket NES on it, and a bunch of NES roms, would they play on a DS?
Will somebody figure out a way to do SNES emulation on the DS? Maybe even N64 emulation?
It's going to be very interesting to see what comes down the line in terms of emulation for the Nintendo DS and PSP.