Google has released the source code for Android 4.2 Jelly Bean meaning developers such as CyanogenMod team will now be able to take the code, have a play with it and of course upgrade their ROMs. Jean-Baptiste Queru of Google's Android Open Source Project posted the details on Google Groups, saying that the Nexus 10 is the best choice for AOSP work on 4.2:
No flagship device so far has been so open, and no flagship device so far has had that level of AOSP support at launch.
Queru also stated that the Nexus 7 3G isn't supported, as Google has been unable to license the GSM stack for AOSP yet, so without it the device won't do more than the normal Nexus 7. Nothing in the AOSP is related to the Nexus 4 either.
Factory images for the "takju" Galaxy Nexus, Nexus 7, Nexus 4 and Nexus 10 are available here. The proprietary binaries to run AOSP on Galaxy Nexus (maguro, toro and toroplus), Nexus 7 (grouper) and Nexus 10 (manta) can be found here. The Nexus S and Xoom will no longer be supported.