Sprint seemed to have gotten the jump on upgrading their LG Optimus G to Jelly bean, but now AT&T will be updating their version of the phone. This has not been made official by AT&T, but users on their community forum pages have mentioned they have begun to see the update appear on their phones. The update is a chunky 336mb, and it will bring your LG Optimus G to Android 4.1.2. So keep an eye on your system updates, because you might have some delicious candy in there soon. Let us know if you have received the update.
Source: Talk Android