I think that was the apu purpose is to become a co-processor like fpu did back then.
The problem is, they say that for PhysX to work properly, you need to have at least 1/3 of the CUDA cores on your co-processor compared to your GPU. Sandy Bridge just won't cut it in this respect. Llano might, but probably only for a midrange GPU. I suppose it could allow some more basic effects.