Oh dang! That man just ordered breakfast in the middle of my rap song!
I could definitely see Natal being used a supplement to FPS gameplay, and really, that's the thing about Natal and more "hardcore" games.
Sure, there'll be the Arcade or Rhythm or gimmick-type games (not a bad thing) that are "optimized" for Natal, where you use all motion-sensing all the time.
But Natal can be used in conjunction with the usual game too: gesture to interact (instead of "hold X"), crouch, etc. And you can still use the normal 360 controller. I'm glad Bungie's expressing an interest in Natal; I just read a hands-on preview of it and it's really looking great.