The new Nexus devices just went live, and, as promised, Google has simultaneously started pushing the code for the latest iteration of our favorite mobile OS - Android 4.2 Jelly Bean - to the Android Open Source Project. Jean-Baptiste Queru, chief Android release engineer, announced the news via a post to the Android Building group.
Update 11:22am: Android 4.2 source just finished replicating to AOSP. "The platform source files for 4.2 have finished replicating, you can now sync." -JBQ
The build number is JOP40C, while the AOSP tags is android-4.2_r1.