Well he is probably going to lose out on his bet, unless we are under tracking America numbers which is possible considering 80%-90% sellouts can be difficult to track as they don't follow normal trends. But I'd still say even with that it would be difficult.
For instance, 4.2 million for the 360 version first month and 1.4 million for Wii version first month (rounded up for the sake of easy comparison). Let's say NSMB Wii does what I'd consider highest potential sales in December. Range would then be 2-3 million for NSMB Wii. Then same for MW2 360 which would be 1-2 million. At the end of December we'd be 5.2-6.2 million for 360 version and 3.4-4.4 million for NSMB Wii. Despite how much ground could be potentially gained it's a long haul in January. Even if we take lowest for MW2 360, 5.2 million, and highest for NSMB Wii, 4.4 million, that is an 800k differential that would need to be made up in January. I don't see MW2 360 selling less than 400k in that month so with the extremes NSMB Wii would need to sell at least 1.2 million in January to tie.
So it would seem this bet is too much of a longshot to overcome. Now once again, I've never been sure on the terms of the bet and I'm sure no matter what happens, Reggie will be able to spin it no matter what. But really this bet is more for PR and hype of both games, namely NSMB Wii, and something we've just taken along for the ride. However, no matter what happens, NSMB Wii will pass up the 360 version WW before that time and I'm sure Nintendo will be pretty happy about that because American numbers only is nice but around the world is going to be what matters to them. That's a nice piece of spin that I'm sure will be thrown out.