I'm guessing he's referring to 360 having mandatory Natal in the box going forward, therefore guaranteeing that every new console sale delivers a Natal sale.
He also seems to be taking the view Arc won't be bundled but will remain a seperate, optional purchase.
If true, then in US for sure Natal should sell more unless people simply stop buying 360s at all or Arc really takes of big time.
But, I think there are still unknowns that easily throw this off.
For one, will a 360/Natal sell for the same price? If so MS are effectively going to be giving Natal away in that case which will hurt profits, if they raise the price of 360/Natal then the price point could reduce sales, or Sony could offer Arc bundles to match, etc. etc.
But still, I can see where he's coming from if he's saying what I think he's saying.
Try to be reasonable... its easier than you think...