While Android 4.2 is now making its way into AOSP (Android Open Source Project), support for one device in that code is not, and won't be for some time: the Nexus 4. Android maintainer JBQ (an awesome guy we quote a lot on days like this) has confirmed that the "Nexus 4 is not supported in AOSP at the moment: no source files, no binaries."
JBQ is not able to comment on why this is the case, or when we might expect the Nexus 4 to join Google's big happy AOSP Nexus device family, though obviously if Google could merge it into AOSP today, they would. So, the only logical conclusion? Something is keeping them from doing that. It could just be that the Android team still has things to finalize, i's to dot and t's to cross and all that, but they're usually pretty on the ball with this stuff.
My guess is that the most likely culprit is licensing. The same issue currently prevents the Nexus 7 3G from being added to AOSP, which JBQ blames on the fact that "[w]e haven't been able to license the GSM stack for AOSP yet." While it's unclear exactly what sort of licensing might prevent the N4 from being added into AOSP, my bet is that we're looking at a 95% chance the answer to that question is, at least in part (if not the entirety), "Qualcomm."
Qualcomm has been notorious for some time as highly protective of its intellectual property, and as the ARM processor game's biggest player - and the leading manufacturer of mobile SoC's - it's pretty obvious why. While we don't doubt for a second that Google is working hard to make sure the Nexus 4 does eventually makes it way to AOSP, it is a bit sad they couldn't make it happen before launch day. We'll let you know if we learn anything more about the status of the Nexus 4's source code.