Last Updated: February 12th, 2013
Earlier today, both the Nexus 4 and the Nexus 10 started receiving small ~1MB OTAs to Android 4.2.1 with fixes to the missing month of December in the People app, among other things. The corresponding open source files are being pushed by Google to the Android Open Source Project (AOSP) as we speak, Android release engineer Jean-Baptiste Queru just announced in the Android Building group.
The build number is JOP40D and the tag is android-4.2.1_r1.
Last Updated: November 14th, 2012
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.