For a studio it's not a good idea to limit their flagship title to high end product of a single vendor.
This is why PhysX is a marketing tool. No ATI, no low/mid Nvidia cards, and high-end cards would have trouble running extensive PhysX on a graphically demanding game (i.e a triple A title). This is why we get graphical PhysX instead of gameplay GPU PhysX.
GPGPU processing is a good thing, but I doubt that at the rate games are advancing graphically, while being displayed on higher resolutions and extra AA/AF/etc, that the majority of a GPU vendor's product line is going to be able to satisfy both the rendering and physics processing requirements. This is no different from ATI promoting DX11 on the 5450, 5570, or 5670 - those cards don't have the power to make full use of DX11.
The reality is that game consumers will be able to spare a core for (lesser) CPU physics much easier than GPU processing power.