Nexus 7 (2012 and 2013 Wifi models) and Nexus 10 users may want to start checking for the big update right about now, as the OTA to KitKat has been released into the wild according to the official Android Google+ account. To accept the OTA as-is, you’ll need to be running the latest stock build from Google, though an unlocked boot loader should pose no problem.
If you didn’t go back to stock so you were ready, you’ve a couple options. The easiest, and recommended, is to sit tight and wait for someone to convert things into a zip file that can be flashed with any recovery, which will follow shortly after the OTA gets into plenty of hands. You could also quickly flash the last factory image, or you could just wait and see if Google releases a new 4.4 factory image.