Good news, ROM enthusiasts: the Android Open Kang Project has released the much-anticipated first build of 4.2. Bad news: it's a completely new ROM, built from the ground up. That means that it won't be quite as feature-packed as you remembered, and device support is limited to the more popular Nexus models, at least for now. You'll also need to install a recovery that's compatible with 4.2 ROMs (the latest ClockworkMod and TWRP should do the trick) and do a full data wipe of your previous ROM. Warm up Titanium Backup.
For the MR1 release, only the Galaxy Nexus (Verizon and GSM flavors), Nexus 4 and Nexus 7 builds are available.