The Cayman GPU works fine with the kexts from OSX 10.8 Mountain Lion, as reported by netkas. This means, the AMD Radeon 6970 and AMD Radeon 6950 work with Hackintosh - making the 6970 the fastest card currently supported by Hackintosh systems. Netkas further posted a patched x3000.kext that will solve the existing problems. Can't wait to try this out myself. Will post updates here.
[Update 2012-3-11] netkas writes about Geforce gtx580 support out of the box. The netkas forum thread 6950 working NATIVELY in 10.8 !!! is currently 18 pages long and there are people getting it to run, some people are still having issues. The 6950 and 6970 running natively with the Mac Pro, as reported in the macrumors forums.