for nes-games, a wiimote is perfectly fine, as it has a near identical button layout that the original nes-controller have.
for snes-games however, a wiimote alone is not enough.

if you connect a nunchuck to the wiimote, will it work on the n900? that wouldnt be a perfect solution (button layout-wise), but at least you could map the buttons in a way that you could use all buttons needed for a snes-game.

(ie analogstick as dpad, c and z buttons as L and R, and the dpad on the wiimote working as a/b/y/x for example)

btw it should be possible to make a classic controller work on the n900.. .that would be the PERFECT solution for snes-games