It's begun - the newest version of Android, 4.2.2, is being pushed to AOSP right now. We saw the accompanying Nexus OTA rollout start last night, with update files slowly springing up thereafter. Right now, 4.2.2 builds can be downloaded for the Takju Galaxy Nexus, Nexus 7 (Wi-Fi), and Nexus 10.
The build number is JDQ39 and the tag in AOSP is android-4.2.2_r1. Here are a few of the many directories that have been updated:
While the push has just started this morning, source will likely continue to show up throughout the day, and we'll update this page as that happens. The Android Building Google Group should have a post by JBQ when the push is complete.
Update: It appears the push has finished, all Android 4.2.2 code should be in AOSP now.
We're working on compiling a full developer changelog right now, and once we've done that, we'll try to come up with a more human-readable one.