updated 01:20 pm EST, Tue February 21, 2012
No hacks required
The developer preview of OS X Mountain Lion already supports some more recent AMD and NVIDIA video cards without any tweaks, according to forum posts by so-called Hackintosh users. Several posts from the Netkas.org forums indicate that an AMD Radeon 6950 running Netkas EFI will work natively in Mountain Lion. One even claims that a 6950 will continue to work after "warm booting back to Lion from Mountain Lion."
Posters at tonymacx86 note that Radeon HD 69xx support is is still rough. One person adds, though, that NVIDIA 5xx cards such as the GTX 570 are also working, "running at full res even smoother than a patched Lion. It's like native." The main caveat with the 6950 is that in the absence of unmodified drivers it may not work properly with an external display; a hack called Chimera has been updated to support Mountain Lion, and should be released to developers within a couple of days.
While Mountain Lion's compatibility with recent video cards doesn't necessarily suggest what will be in upcoming Macs, it does at least show that Apple is already using a comprehensive set of drivers. Apple may launch new Macs in the summer, the same timeframe as when Mountain Lion is scheduled to ship.