From what I have noticed, Apple's Pro apps are mostly CPU dependent. Unless there were a mass of code that the GPU's would run noticeably faster it wouldn't be worth the manpower/time.
Honestly SLI would be cool. It would have to be available in a machine that was less than $2500 (read xMac). I don't know why they don't offer SLI in the iMac, I would accept a thicker unit if I knew it was packing 8700/8800M GT SLI.
The ones i use are, but Motion is actually based a lot on the GFX chip in the machine. I am sure that SLI and Crossfire won't benefit anything that isn't programed to use them but it could happen. And now that Final Cut Pro has some Motion features in it, that app could benefit, and Shake should be updated sometime in the near future, and that app would be very GFX dependent.
This is all speculation of course, so I am still just dreaming. The other Apps are a toss up, but i know that After Effects is GFX dependent as is Illustrator and Maya.