In true Nexus fashion, Google has released the system images for a bunch of Nexus devices. If you ever brick your phone, or just want to start over, these images will be there for you. The images are 4.1.2, so they’re actually newer than the software many devices are running.
The devices that get the images are the Nexus 7, the Galaxy Nexus (takju and yakju), and the soju Nexus S. They have also released a 4.0.4 system image for the Nexus Q, which is a first. If you ever have any problems, you can fix em up with just a quick flash.