I don't think they will do that, Ian. The reason is because they didn't do that with the HD-DVD thing. If MS had actually built it into the 360 (or at least bundled it with new systems) then there would have been a far better chance HD-DVD could have won the format war awhile back, but MS didn't do that. Maybe its just because they were afraid the format would fail (which it did) and didn't want to be dragged down with it, which is possible, but I think it was more just an oversight. I don't think they really grasped the difficulty in selling the peripheral model and probably seriously thought they were doing things right. So that's what could happen here with NATAL as well. Hell, maybe MS is afraid to build it into their system because they fear it will fail? That might actually be something they're worried about, but if it isn't then the other reason they might leave it out is because they just don't know any better and think it will sell well as an extra peripheral.
If Sony/MS treat this as their "next gen" then it might work out, but odds are they'll just pull a Sega and then announce the Xbox 460 and PS4 before those addons have a time to really catch on, and then consumers would just save their money causing the addons to flop. What you're suggesting does make sense from a logical standpoint, but I just have my doubts that MS/Sony will do that. Sega didnt' follow that road, and there's no reason to believe the people running these companies are any smarter than Sega was back in the mid-90s. Remember how MS had to beat everyone else by launching the 360 a year ahead of the competition? Well, that strategy worked out well for them because they are leading the PS3 right now, so they will look at that strategy as a success and probably want to repeat it by getting the 460 out ASAP. Remember, their console has been out for about 5 years now, so for them they probably sense it is time. Not to mention the fact the 360 isn't quite powerful enough to handle NATAL fully.