PhysX seems to have a lot of stability problems in shipped games. I'm sure with time it'll get better, but ATI's response was really nice:
they showed a prototype of Havok implemented in OpenCL.
Now, the kicker is that OpenCL is an open specification, so NVidia and others can and probably will implement drivers for it, then you'd write OpenCL programs once and run them on either a GPU or a multicore CPU, whatever's best in a given configuration.
But we're not there yet. So don't think about it too much
Now, if someone implements Box2D in OpenCL...