It's difficult for Apple to justify supporting two concepts designed for gaming... when they don't have GAMES.
Oh, and when only one computer would be able to truly take advantage of this, and that one computer cannot physically support
either technology as shipped by Apple, and that one computer is not sold to an audience that would give a crap.
This is absolutely true...
For either solution to work properly (render frames correctly) they need driver profiles for EACH game.
For either solution to offer any value, you need to have two identical cards that when combined offer more power than a better single card solution.
Think about this last sentence... what two cards can you combine, and in what system, that when paired together, offer more power than an available single card solution? NONE!
The only system that can possibly support this, is of course the Mac Pro. The only cards you can combine in that system are two GT120's. Even 4 GT120's in QUAD SLI won't outperform a SINGLE 4870. The Mac Pro can't support dual 4870's (lacking power connections).
This is just a couple of good reasons why Apple, Nvidia, or ATI won't be supporting either of these solutions any time soon. Keep in mind it's not just an Apple decision... Graphics card manufacturers would have to be motivated to provide capable drivers and/or license it. The market for this technology is probably .0001% of the total GPU market... do you think they care about a market that size?
Perhaps the most compelling clue is the ridiculously minimal choice we have for graphics cards... Apple, Nvidia, and ATI simply don't take the MP market seriously for high-end graphics... if they did, we would see higher-end cards more often for this platform. Apple most likely has to PAY Nvidia and ATI to build cards for the Mac Pro. If they didn't have to pay and both companies saw this as a lucrative market for them then every card both companies offered would be available in a Mac Pro version. ATI and Nvidia have much bigger markets to worry about than ours.
One last comment... be careful what you wish for. One of the appeals of OSX and Apple computers is their rock-solid stability. Vista was largely such a flop early on because of terribly poor driver support (particularly from Nvidia and ATI) despite the OS being in beta for nearly a year! SLI and Crossfire are also both flaky as hell and most people that have used either will never do so again. Do you really want this on OSX?