users of the GTX 285 have been complaining about a power management issue in OS X that prevents the GPU from running at full speed for CUDA apps.
I just found out this also happens to the GeForce 9600M GT.
Under normal use the 9600 runs at 0.34 GHz, and at that clock pinned device to host copies are around 1.5 GB/s.
Running some OpenGL code makes the clock go up to 1.25 GHz, and pinned copies double to 3.0 GB/s.
Happens on Snow Leopard too.