Macbook Pros currently running Leopard (10.5) utilize only one graphics card at a time (a decision disguised as a power profile change). Will OpenCL (introduced to the platform in 10.6) continue this trend by only allowing access to the active graphics card, or will it be able to use resources on the 'idle' card?
Found on Ars Technica (LGT Source Article):
"Furthermore, an interesting tidbit about Snow Leopard's implementation is revealed by early tests. Though Snow Leopard doesn't seem to enable dual GPUs or on-the-fly GPU switching for machines using the NVIDIA GeForce 9400M chipset—a limitation carried over from Leopard—it does appear that the OS can use both as OpenCL resources simultaneously. So even if you have the 9600M GT enabled on your MacBook Pro, if OpenCL code is encountered in an application, Snow Leopard can send that code to be processed by the 16 GPU cores sitting pretty much dormant in the 9400M. The converse is not true, though—when running a MacBook Pro with just the 9400M enabled, the 9600M GT is shut down entirely to save power, and can't be used as an OpenCL resource.
tl;dr - It works as I hoped if you're using the 9600M GT, but if you're using the 9400M the 9600M GT is completely disabled.