The Jelly Bean 4.1.1 images for selected Nexus models have been released by Google, allowing you to roll back to a safe, factory state no matter how bad you've f&%^#d up your Nexus. The particular models are  --

This is good news for all of us who hack and modify our Nexus devices, as having an easy and safe way to return to a booting device is the first rule of Android hacking. You can grab the images at the source link, and be sure to hit the forums for more information about how this all works if you have any questions.

Source: Android Developers; via Android Central forums