I just thought about a different approach that anyone mentioned in this thread (I haven't take a read at all the thread but I think no one put this into discussion yet)
Let's imagine than in the next pricecut the 360 will be at 200 bucks with HDD and Kinect and Adventures. All bundled, and there's no other sku. From that point on, whoever wants a 360 will have a kinect and KA too.
That way, it will not only reach 20M but it can go further too.