Nvidia's Physix won't work in a mixed video card environment.
This what I heard as well, which is really lame IMO. It's not the exclusivity that bothers me but the fact that you can't use it even if you paid for it. I would love to run a beefy 7870 alongside my 560 Ti for physx, cuda, or even switching for a game in case of faulty driver support.
Intel Core i7-875K, Asus P7P55D-E Pro, Win 7 Home Premium
MSI GTX 560 Ti OC, Mushkin 2x2GB DDR3-1333, Corsair TX650
Cooler Master Hyper 212+, Logitech Z-2300, ASUS Xonar DX
Samsung Spinpoint F3 1TB, Dell Ultrasharp U2410, Antec P183