There’s two shortcomings…
The first is you could throw away ATI HD 2400/2600 on iMac, ATI HD 2600 XT on Mac Pro, in fact any ATI card except 4850 & 4870 found in latest iMac and MacPro. Naturally any GeForce 8000 series, 9000 series or GT/GTS/GTX is compatible with OpenCL & CUDA.
I thought that ATI has problems to implement OpenCL due to lack of supporting hardware, and I suspect that even on 4000 series, performances on GPGPU (General Purpose computing on GPU) will lag behind those found on nVidia’s due to lack of “real” Shared Memory.
The second short coming is that I didn’t see any information about support of CPU and multi-core CPU on OpenCL. OpenCL is not designed to make software that run on GPU, but to enable software to run on *ANY* modern GPU and *ANY* CPU too, too unleash the full potential of a computer.
I will have to check for it, and report numbers this week-end 
