Implementing this controller now would be a colossal failure in my mind. Putting it in now, in my mind, wouldn't result in a high attach rate, which results in no support, and a big waste of money. I think that if they wanted a motion sensing controller, then they should wait until their next console.
However, if they did want to implement it now, then they should pack in a controller with some small scale games like the arcade pack, that uses this new controller and maybe with a points card (maybe 400 points) or a month of gold, and hope it sells.