People asking for sli, you're asking for more trouble than anything else. SLI doesn't work with pro apps like Maya, even on Windows, and it is the cause of many apps acting flaky. Google Photoshop SLI problems. You don't need SLI for pairing cards for OpenCL or CUDA. I use Octane with dual NVIDIA cards in OS X.
Considering that no one buys a mac pro for games, the odds of Apple picking a gaming feature over pro app stability is nil. If you want two high power cards in a mac pro, google my Ars Technica review of the mac pro 2010 where I show how to use a y splitter to power two cards without problems. I am running the Quadro 4000 and Radeon 5870 this way with no problems. I'm at gym posting from phone so not convenient to get links