Do you have it set up correctly in forceware? Go into your nvidia control panel, and make sure that on the PhysX page, you manually set which card you want to handle PhysX, don't let it automatically decide. Also, in the game, make sure you DO check the "hardware physics" box in the video screen, otherwise the CPU will do the PhysX processing.
I haven't actually checked my GPU usage with monitoring software, but I can definitely tell it is working (I just added the 9600 gt as a physx processor the other day, and I noticed the difference immediately). When I get home tonight, I will actually check the usage and see if it's working, although I'm pretty confident it is.