From my searching, I am pretty sure NES and SNES controllers operate off a parallel interface and not a serial interface.
If you do manage to get the controllers to operate via serial, you do have some driver options... namely the Zeta gamepad drivers and the Gravis stinger gamepad drivers.
But from what I recall in the experiments people were trying, they were unable to get a gravis gamepad to operate on a Axim X5 through a serial cable with and without the max232 chip in the line.
The only real ray of hope I see is in this IR pad or
http://www.gamestop.com/product.asp?product_id=801842 bluetooth pad. The reason for this and not the one they showed off at Cebit or whatever tech convention is because they actually have the product out. The only problem is we dont have drivers nor do we have any idea how this interfaces. But considering that they said its bluetooth, we can hope its a bluetooth dongle on the xbox end and normal HID driver support unless they are cruel and unusual and use a properity bluetooth data xfer system of which I dont think will qualify to be bluetooth.