Stop the Nvidia hate already. You are reading that statement wrong:

"Can I use an NVIDIA GPU as a PhysX processor and a non-NVIDIA GPU for regular display graphics? No. There are multiple technical connections between PhysX processing and graphics that require tight collaboration between the two technologies. To deliver a good experience for users, NVIDIA PhysX technology has been fully verified and enabled using only NVIDIA GPUs for graphics."

I've bolded the important part. That's their reason behind it. Just to make you understand: physx is just as much about hardware as it is about software. For example, physx may work now with the current ATI cards and drivers. But what if, let's say, a new ATI driver or card breaks this compatibility. What should Nvidia do then? Invest money and time to fix something their main competitor broke? Why should they do that? And why should they help their competitor in the first place? Physx is a selling point for Nvidia hardware. Why should they share that with ATI? If Nvidia would allow right now, through a new driver physx to work with ATI cards they would instantly loose sales as people would upgrade to ATI cards and not Nvidia. I don't know how many people are held back on the green team by physx but if it's only one guy, in Korea somewhere, that would buy a 5870 instead of a GTX470 and keep his 8800GT for physx, they would loose a couple of hundred dollars to ATI. Why would they do that? Aren't you asking much of them here?