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Basically you can now have a AMD primary and Nvidia PhysX dedicated with this driver, no hacks.

http://www.anandtech.com/show/3744/nvidia-forceware-257-heterogeneous-gpu-physx-its-a-bug-not-a-feature

and you should be able to get it here :

http://www.ngohq.com/news/17896-nvidia-forceware-257-15-a.html

Nvidia have probably re-released the driver with the “protection” in place, so this is probably one of many places to get it.

A vid of it in action (in German text) :

http://www.youtube.com/watch?v=BRikKybcno0&feature=player_embedded

No need to hack the drivers for this one ;)

source: http://www.semiaccurate.com/forums/showthread.php?t=2612

update: nvidia re-released the drivers as seen on a recent nvidia blog post:

A lot of you have been asking about PhysX and the 257.15 beta driver we posted on Monday. First off it is true that PhysX is enabled when running on NVIDIA GPUs when AMD GPUs are used in the same system. PhysX is a compelling technology that makes PC games great – I am not surprised our fans are eager for it. 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.

source: http://blogs.nvidia.com/ntersect/2010/05/update-on-release-256-physx-support-1.html