If you're an owner of a T-Mobile Samsung Galaxy S III, then you can now pick up a leaked version of the Jelly Bean update. XDA user LuffarJoh posted up the leaked ROM for users to install on their phones, but judging by some of the bugs and other issues with the ROM, it would appear to be an earlier version of the ROM.
To upgrade, users are required to be running the stock T999UVALH2 build. Furthermore, you may lose root when applying the update. These are just a couple of the caveats. The ROM appears to have bugs with Wi-Fi calling, stock browser and video player. Google Now may or may not work and there could be some general performance or lag issues.
That said, it's always nice to get upgraded to the latest OS, especially if it's ahead of the official launch. If you can deal with these bugs and want to try out the operating system for yourself, you can pick it up at the XDA forum post. Just make sure you're willing accept the bugs and potential risks before upgrading.