Lik-Sang sells a hardware NES emulator for the GBA. Looks ugly, needs extra batteries and probably has bugs, but seems to work if you really want to subject yourself to that.
The legality is questionable (though I'm not sure if there really is a law against it), but at least you need to feed it with real NES cartridges, so it doesn't unnecessarily promote piracy (or violate Nintendo's patent on handheld emulation, yes, they really did patent that).