If you like to live dangerously, and can't wait any longer to get Android 4.1 Jelly Bean on your Galaxy S III, you'll be pleased to hear that a (very) early preview build of CyanogenMod 10 is now available for Samsung's 2012 flagship. The build isn't yet feature complete, but it seems to be about as functional as the current batch of CM9 nightlies of the S III. That is to say the current build includes a couple of known stability issues with the camera and Wifi tethering -- the kind of thing you'd expect from a nightly-level build. Also, this build is only for the international Galaxy S III -- that's the one with 'GT-i9300' listed under Settings > About device > Model number.
So it's not complete just yet, but it's an early taste of Jelly Bean on the Galaxy S III, and early reports suggest it's very quick and surprisingly stable. If you're feeling brave, you'll find flashing instructions -- and some disclaimers you should definitely read -- over at the source link.