Nexus 7 owners, it's your turn. Earlier this evening, before many countries even had access to Nexus 4 or 10 orders, Google began pushing the 4.2 (JOP40C) update to the Takju Galaxy Nexus. After some patient waiting, it appears the Nexus 7 is now ready for an update as well. In case you don't feel like waiting on the OTA though, we've got the download link and some super-easy instructions for manually updating your favorite 7-inch slate.
Prerequisite: Your Nexus 7 should be running Android 4.1.2 build JZO54K. You will not lose your data using this method.
Download the update zip from Google's servers (MD5: 7bee9b220c20979763af563cad56af94)
If you have a custom recovery, you can just copy it to the device, reboot to recovery, and flash directly as you would any other zip. You're done.
If you have the stock recovery, carry on.
Reboot your Nexus 7 and hold the VolDn button while it's booting up. Once you see the fastboot menu and the word Start, press VolUp a few times until you see Recovery and then press the Power button to enter recovery.
You will see an Android with a red exclamation point. Now press VolUp+Power together, and you should see the menu below.
Select the 2nd option called apply update from adb.
Connect the Nexus 7 to your computer with a USB cable.
Make sure you have a recent version of adb that supports the sideload command (if you don't, download the latest Android SDK to ensure your adb is up-to-date).
Keep the file you just downloaded on your computer and open up a command prompt (cmd in Windows, regular shell in Linux and Mac) in the same directory as the zip file.
adb sideload 094f6629314a.signed-nakasi-JOP40C-from-JZO54K.094f6629.zip
You should see the progress bar go up to 100%, like so:
sending: 'sideload' 100%
At this point, the device should have started installing the update.
Reboot - you're done. Enjoy Android 4.2 on your Nexus 7!
Update: In case you're wondering, Gmail 4.2 with pinch-to-zoom and multi-user support are included. Here are some screenshots.