Google started rolling out the Android 4.2.1 update yesterday, for the entire Nexus family, and not making the Android development community wait, Google has also made available the Android 4.1.2 sources on the Android Open Source Project (AOSP).
The build number is JOP40D and the tag for the 4.2.1 source is android-4.2.1_r1, and it is now available for syncing from the AOSP repository. The general public won’t exactly have any use for the sources directly though, it is only good for the developers who make custom ROMs, so your favorite custom ROM should soon see a version based on Android 4.2.1. Google should also be posting the factory images for all Nexus devices in the coming days.
Android 4.2.1 is a minor update and brings along changes such as fixing the missing month of December in the People app, support for HID gamepads/joysticks over bluetooth, and a few other bug fixes.
Source: Google Groups