I can't speak about the 7000 series cards, obviously, but I am currently running a 6850 and 6870 in Crossfire. OS X (10.7.2) detected both cards, loading up the necessary frame buffers and whatnot. They work flawlessly for me. I had been using them to drive three monitors, which worked quite well. In OS X though, Eyefinity doesn't work obviously, so the three monitors are treated independently and you won't be able to run games, even those supporting Eyefinity, across all three. Also note that OS X can't harness the extra power that your primary GPU would gain running them in Crossfire (or SLI). OS X will see them as two independent GPUs.