If you like to roll your own built-from-source Android ROM -- or if you rely on a developer to roll one for you -- rest easy knowing that Android 4.2.1 (Build JOP40D) has dropped into the Android Open Source Project. That means the code is there, and proper building can commence. No huge changes here (the missing December thing is the most notable at this point), but having the code is better than not, and Google's pushed it out just hours after devices started getting the update.
Google's Jean-Baptiste Queru also mentions that factory images should be pushed "in the next few days," so you can get your reset on without worry.
Source: Android Building group