This is actually a Problem with all Camera software. It's normally made for the Lighter...whiter people in mind. Normally your face has to be on the lighter side for facial recognition to happen. But this would also explain why the people in the Demo vid for Natal...were all white. I mean camera's with blink detection normally get an error when an Asian takes pictures.
Its really something Microsoft would have to invest A LOT on to work the kinks out of. Unless they go along with Sony's plan and release a controller along with it.
Read the article, but even they point out something:
Unfortunately, that may not be the best evidence, as NeoGAFfers spotted edits in the video -- the in-game footage of Burnout Paradise shown while Willie Gault is playing is identical to that shown during Zachary Levi's play session.