I think Natal will help re-coup some of the console sales losses they had against Sony from this xmas all the way till It's release, but other then that I don't see it taking off as much as Microsoft wishes. It should keep Microsoft afloat for second place till the 720 is out regardless. If MS thinks third place is a real bad thing and the 360 doesn't sell with another price drop and Natal then look for a 720 out in 2 years rather then 3-4+ everyone has been predicting.
It's just that simple.