Sammy has spat out the source code for GPL portions of the AT&T and T-Mobile Galaxy S III firmware, and made it available for download at their developer site. It goes without saying that unless you're the type who builds software, this won't be something you need to download, but for the folks who do it means things like custom kernels and tweaks, which a good many folks love.
No word on any other versions, but since they haven't shipped yet we're not going to give them any grief about it just yet. If you're an interested developer, or just curious what a bunch of Linux kernel source looks like, grab either (or both!) versions at Samsung's open-source developer portal below.