Well, I actually see what Microsoft's plans are IF they are smart and looking towards the future.
Project Natal must be viewed as an add on device like all other add on devices...which given that track record would hint that complete failure. However, if the idea is to introduce new types of gaming and hopefully take some thunder and hype away from the Wii it may be a good Trojan horse leading into the next generation.
Why?
Because the next generation Microsoft can make Natal compatible with the new console and have motion control ready to go without increasing the cost of the console too much, and keeping the core Xbox 3, a gamers console. By that time Natal may be cheaper to produce allowing for a bundled system with the console, or a cheaper one for those who already made the investment.
Will this concept work? Maybe...though truthfully I have my doubts, because if initial reaction to Project Natal is poor for any reasons, then seeing it apart of the new console will be detrimental. However, the reverse is not true...seeing Project Natal as being compatible just seems like something that should be done.
Just like I expect the Wii accessories to be compatible with Wii 2, even though I expect a new (better) motion controller to be released with the system. I expect the classic controllers, Wii Balanced Board, and such to still work.