No one, as others have said. MS must have some projections, but there are a lot of variables to consider, not least the historic performance of standalone peripherals with specialist games (not too great) vs the success of the Wii (a console on which the motion controls are not a peripheral but the main control scheme).
I think it'll do okay, but will need some serious luck, good timing to market and fantastic marketing to have even a chance of going gangbusters - but it's just very hard to call because I feel this situation hasn't happened before so historic evidence is scant.
By the above I mean a heavily marketed peripheral hasn't shipped on a popular console after another, even more successful console, has popularized the idea of a particular method of controlling games.
Try to be reasonable... its easier than you think...
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