If some reports are correct, Natal uses around 20% of the processing power of the XBox 360 in order to perform its functions. With all due respect to the Wii, I rather feel that a loss of 20% of processing power would be a much bigger hit for it than for the XBox 360.
I know it's inapt- they're very different platforms, a certain amount of the XBox's power goes into making its HD visuals versus the SD ones of the Wii, and so on, and so on. Still, the most sophisticated perephrial addition the Wii has gotten to date is the Wiimotion Plus. I'm genuinely not certain the Wii would have enough cycles left to process an enjoyable game if it was trying to track four peoples' full-body movements.