Yesterday, Android 4.1.2 was released for the Nexus 7 and the source code was merged into AOSP. It has taken the CyanogenMod team one day to update their custom ROM to the latest version. It begs the question that if a group of amateur enthusiasts can manage this, why does it take OEMs months, if ever to update Android versions?
Anyway, rants aside, you can download the latest nightly for your device here, or if you are running one of the more recent nightly builds, you can get the update through the new-fangled CyanogenMod Updater.
