NVIDIA commented on their nTersect blog that support for Hybrid PhysX in the GeForce 257.15 beta driver was not intentional. The company now claims that they can't offer Hybrid PhysX due to quality assurance issues..
When using this beta driver no additional hacks are required to enable PhysX. While it was not intentional, due to the overwhelming positive response to the beta driver we have decided to leave the beta up with support enabled.
The fact remains that the investment to do a full QA cycle on this AMD/NVIDIA hybrid GPU configuration is beyond what NVIDIA can support at this time. The cost of maintaining AMD/NVIDIA hybrid configurations is larger than the development expense for PhysX, and I do expect that there are games and configurations where this type of system just won’t work. Please, don’t call me :>. Therefore our WHQL certified drivers will continue to have this support disabled.