the 360 has been out for over 5 years now and reached affordable pricing at $200. the 360 will of course sell for some time to come but old trends (which i know aren't 100% applicable this gen) would suggest that the 360 will keep going down YoY until it's out of the market. because natal could drastically change the type of experience the 360 has to offer i would say it is the only hope MS has of reversing the inevitable decline.
same goes for the ps3, it has a great year last year but this year it will be down YoY imo.