These guys work fast, don't they? After "noticing some demand" for third-party CM 10.1-based ROMs on the new Nexuses, the team decided to go ahead and start rolling out official nightly builds, beginning with the Nexus 4 (mako). This is the first official CM 10.1 build to be released thus far, and is currently based on 4.2, as 4.2.1 just hit yesterday.
The team noted on G+ that builds for the Nexus 10 (manta) should be the next on the list, and will roll out "as soon as it's ready." All other devices will continue to get CM 10 (Android 4.1) builds while they finishing merging the 4.2 code.