It appears that Halo: Reach may be a warm-up exercise for Natal sales. Bungie Studio President Harold Ryan tells the Seattle Times that he "absolutely" thinks that Reach could use Microsoft's motion-sensing peripheral (provided both launched around the same time frame).
Let's take a look at why Project Natal + Halo makes sense:
The Halo name has helped sell other iffy propositions like Halo Wars.
The game is already called Reach, come on.
This answers the question: "How is MS going to get core gamers to buy/upgrade to Natal?"
However, then we must ask: "Do core gamers want to actually exercise with their Halo?" This gamer's biggie-sized core politely responds, "As long as it involves fries and a shake after."