I am also curious about this. I did some research and found some interesting articles. It looks like OS X 10.5 can't use both GPUs at the same time, but perhaps windows can in SLI mode. There is much talk about how the new implementation of Open CL in OS X 10.6 snowleopard would take advantage of multiple GPUs. But I don't think there are many OpenCL apps yet. check out these articles.
http://forums.macrumors.com/showthread.php?t=582405
http://www.macrumors.com/2009/08/30/ope ... -pro-gpus/
the DVX codec resolume uses takes advantage of the GPU to decode video. I wonder if one GPU could process video effects while the other decodes video. That would be cool.
But it does not look like the Macbook pro will allow you to do any general graphics processing with both cards. OS X requires a logout to switch between cards.
