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. From what I can tell, a lot of the code is already up - here are some links to get you started:

The source code for 4.2.1 is being pushed into AOSP right now. This is a minor release on top of 4.2. The build number is JOP40D, the git tag is android-4.2.1_r1. 4.2.1 uses the same device-specific proprietary files as 4.2. The factory images for the various devices will follow in the next few days. JBQ

Update 11:23am PST: The push is done - fire sync your git engines:

The git replication has completed, feel free to sync. JBQ

Update 11:50am PST: And here's what's new in Android 4.2.1.

Source: Android Building