Update: Multiple confirmations now that this update is indeed happening. Commenters and some well-known XDA users are getting JWR66Y. It's still looking like a minor security update and nothing more.
Update 2: The 3G Nexus 7 (2012) is getting the update as well. Not only that, the OTA update link has been spotted. This is the Nakasig version, remember. The WiFi variant is Nakasi, and no news on that yet.