There's not enough information available presently to definitively address this concern, however, I suspect that most modern Macs with discrete GPUs will have no trouble with OpenCL, including your iMac. The 8800 series GPU, though eclipsed by newer GPUs, is still a very stout GPU.
Also, do keep in mind that software must be written to take advantage of OpenCL, and I suspect it will be at least a few years before we see a plethora of the larger commercial apps re-written to do so. Considering how long it took Adobe and Microsoft to natively support Intel Macs, it might be a very long time indeed.