The Android 4.2 update is currently rolling out to the most members of the Nexus family. Some may be wondering how long they have to wait before it’s possible to root their updated device. The answer is “right now.” Like with most updates, Android 4.2 can break root if you don’t take any precautions to prevent that from happening. However, the magical tool known as the Nexus Root Toolkit can easily re-root your updated Nexus even though it hasn’t been officially updated to support Android 4.2 rooting.
If you’ve used this tool before, you should already be familiar with how easy it is to use. The only change you have to make involves changing your model Nexus 7 model type to “Android *** -Any Build.” From there, it’s as simple as clicking the root button.
In my experience, the process wasn’t completed automated. I did have to manually select a couple zip files on the Nexus 7 by following the toolkit’s instructions, but it was nothing complicated. Full root was still achieved quickly.
Source [XDA Forums]