Android maintainer "JBQ" just broke the news that Android 4.1.2 was being released into AOSP today, and now we're hearing that a matching OTA update is being pushed to the Nexus 7 as we speak. It's 31.3MB in size, and the build number JZO54K. If you're not finding the update when you check, don't worry - it'll get there. The rollout is likely staggered and limited to a smaller group of devices to start.
Our I/O Nexus 7s just received update notifications that a new software build is available for download - this is very likely the final retail build users will see when they receive their devices.
While a "changelog" of sorts pops up with the update notification, it's stock dialogue from a full-on Jelly Bean upgrade, not something specific to this particular update for the Nexus 7. More than likely, this OTA just irons out some bugs and turns off some unnecessary data logging and pre-release testing tweaks.