It depends. Maxwell looks like a good candidate for the Mac Pro, but I don't know if Apple could ship a Mac Pro with a bridge that works for both Crossfire or SLI. At the end of the day, it's just pins, so it may not matter, but if the pin count is too small it could be an issue.
I don't think Apple will want to change the pin count for the card attachments between revisions, especially if they want to do an upgrade program for older Mac Pros (which I put at 50/50 chances right now.)
I also don't think Apple would add an Nvidia card just to appease CUDA users. Apple doesn't care about CUDA, and if it came to it, I'd see them suggesting using an Nvidia accelerator over Thunderbolt or something.