NVIDIA Forceware 257 & Heterogeneous GPU PhysX: It's a Bug, Not a Feature
 

We just got done talking with NVIDIA about the matter and they clarified the issue for us. In what we expect is going to be a disappointment for many of you, the lack of a PhysX restriction on the current 257.15 beta drivers is a bug, not a feature - the restriction should have been in those drivers and it was not. NVIDIA will be reinstating the restriction in new downloads of the beta driver and in the WHQL build of these drivers.

NVIDIA tells us that they will also be "fixing" the 257.15 beta driver on their site, so new downloads of that driver will have the restriction in place

http://www.anandtech.com/show/3744/n...-not-a-feature