NVIDIA has reportedly removed the driver-level code which restricts users from having an NVIDIA GeForce GPU process PhysX with an ATI Radeon GPU in the lead, processing graphics. Version 257.15 Beta of the GeForce drivers brought about this change. Possible commercial interests may have played NVIDIA's previous decision to prevent the use of GeForce GPUs to process PhysX with ATI Radeon GPUs, where users could buy an inexpensive GeForce GPU to go with a high-end DirectX 11 compliant Radeon GPU, thereby reducing NVIDIA's margins, though officially NVIDIA maintained that the restriction was in place to ensure Quality Assurance. The present move also seems to have commercial interests in mind, as NVIDIA could clear inventories of GeForce GPUs at least to users of ATI Radeon GPUs. NVIDIA replenished its high-end offering recently with the DirectX 11 compliant GeForce 400 series GPUs.

Update (28/05): A fresh report by Anandtech says that the ability to use GeForce for PhysX in systems with graphics led by Radeon GPUs with the 257.15 beta driver is just a bug and not a feature. It means that this ability is one-off for this particular version of the driver, and future drivers may not feature it.