Because of the Samsung Nexus S' VGA front camera, face unlock was left out of the official and AOSP builds of last year's reference phone. You know where this is going -- the files have been yanked out of the Galaxy Nexus and face unlock is working in all it's gimmicky goodness. It's even working with a beard and glasses.
It goes along great with a pair of AOSP builds and a few tweaks for the Nexus S twins, which are available as well. This is why we love Android, open-source, and Nexus phones. Ice Cream Sandwich is running better than most expected on year-old hardware, and we're having a blast playing with things. We have the phones, we have the code, let's tear the roof off this mother.
Be sure to grab the right version for the right phone below.