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. 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.
Last Updated: December 19th, 2012
If you're looking for the Android 4.2.1 update to the Nexus 4 that started rolling out this morning (it's a very minor patch, at only 1.1MB), you can find it, straight from Google's servers, at this link.
The update, JOP40D, so far has one known fix: the missing month of December in the People app (Contacts.apk). Presumably it fixes other things, too - Google probably wouldn't release a patch just for a single bug, but we haven't figured out what else this patch may address just yet. That said, there is no shortage of things it could (read: should) address.