I suspect that divergent interfaces like Natal, Arc etc would mean more platform exclusives rather than fewer as developers try to be first to take advantage of them on the core console platforms.
If Arc > Natal then the PS3 will get more platform exclusives.
If Natal > Arc then the Xbox 360 will get more platform exclusives.
As of right now I hear they have 16-18 titles coming for Natal and are spending up large once again. So the exclusive 3rd party titles are probably back on the way in for the Xbox 360 at least.