CyanogenMod 10 is still cooking, and heck, CM9 isn't even fully baked for all devices yet. That hasn't stopped Steve Kondik, aka the guy who puts the Cyanogen in CyanogenMod, from posting preview builds of CM10 over on XDA for the AT&T and T-Mobile Galaxy S III variants. Plus, a third preview build being posted for the Sprint version by another developer. Keep in mind, of course, that "preview build" translates into "absolutely nothing is guaranteed to work." It is exciting, nonetheless.
The threads go out of their way to point out that CM10 isn't even close to being ready to ship and that just about everything that could go wrong probably will. But if you clicked on a link that said "preview build" and are seriously considering flashing it, that doesn't scare you off, now does it? You can grab the ROMs from the source links below if you're feeling bold.