Im sure that when Natal is released it will come bundled with all new 360 consoles (perhaps as a way of them not having to further drop their price). I am also expecting MS to encourage existing owners to purchase Natal by offering it for maybe £60 with a bundled "Wii Sports" mini game collection. This should still make Natal itself profitable but also help build hype / momentum - if existing owners ignore it then it will struggle to build momentum.
I also see MS making a lot of money from Natal long term by licensing the software and technology to other companies. As people have already suggested TV's, PC's, etc.