Nvidia has cut the cord on folks who are using its GPUs to accelerate PhysX effects alongside an AMD card. Nvidia's newer drivers deny the ability to use hardware PhysX acceleration unless your GeForce is handling the graphics as well. A forum post on NGOHQ shows an email explanation from Nvidia's Customer Care, which lists a variety of causes for the decision, including "development expense," "quality assurance," and "business reasons."
The Tech Report notes that while this applies to GeForce 186 drivers and beyond, it only really affects people running Windows 7, as Vista's Windows Display Driver Model (WDDM) 1.0 can't run multiple GPUs with different drivers simultaneously -- as opposed to Windows 7's WDDM 1.1, which can. The site also spoke with Nvidia general manager Ujesh Desai, who cited technology and compatibility conflicts as the motive for the limitation.
No matter Nvidia's intention, can you blame them?