AOSP maintainer Jean-Baptiste Queru has announced that Android 4.1.2 has been to Google's Android Open Source Project repository. A changelog is not yet available, but the source release comes ahead of a planned rollout to Nexus 7, Galaxy Nexus and Nexus S devices.
The original announcement of the JZO54K release (wearing tag android-4.1.2_r1) can be found in the android-building Google Group.
The Nexus 7 and Galaxy Nexus MoDaCo Custom ROM releases will be updated as soon as possible. :)