http://code.google.com/android/nexus/images.html

This file, once unarchived, contains the bootloader, baseband, and the
rest of the system.

You'll need to have fastboot in your path, and to have the device in
fastboot mode before you can use it (either "adb reboot bootloader",
or press-and-hold both volume-up and volume-down then press-and-hold
power).

For convenience, a flash-all.sh script is provided as well, which
restores the entire phone to its factory state.

If you're only flashing some of the images, I strongly recommend
flashing things in the same order as flash-all.sh. Specifically, flash
the bootloader before the radio, and reboot after flashing the
bootloader or the radio.

Don't forget that after flashing back to a factory state, your
bootloader is still unlocked. Don't forget to lock it back in order to
secure your device ("fastboot oem lock").

Hopefully this'll be useful to people flashing custom AOSP builds, as
it provides a clean supported way to return to factory state.

JBQ

--
Jean-Baptiste M. "JBQ" Queru
Software Engineer, Android Open-Source Project, Google.

Questions sent directly to me that have no reason for being private
will likely get ignored or forwarded to a public forum with no further
warning.