An initial update to build JWR66N and subsequent update to build JSS15J are available
Early adopters of Google's new Nexus 7 are being greeted with software updates right out of the box. The first update will automatically begin as soon as the tablet connects to WiFi for the first time, usually during the initial setup process. The tablet will shut down, boot into recovery, apply an update to bring the build number up to JWR66N, and bring the user back to the setup screen to continuing signing into a Google account.
The second update comes in a more traditional fashion: it is downloaded in the background, and then triggers a system update notification in the notification tray. Once the user accepts the update and it is applied, the build number becomes JSS15J. Both seem to be minor updates, most likely containing some system fine-tuning and bug fixes.
Now if Google would just post the factory images for this model.