I noticed this too, not that I really miss it. I suppose the NS FFC has a tenth of a megapixel lower resolution, but I doubt that's why it was left out. Beyond that, I'm not really sure why the NS would be any less able to run it than the GN. Maybe the camera hardware is worse enough in other aspects as to make it impractical. Maybe the Google team decided it was too gimmicky to include, or were otherwise put off by the buzz over the feature's vulnerabilities. Or maybe the Android team just got lazy. :P
...Anyway, I'm not worried about it. In practice I don't think Face Unlock is much faster or otherwise more practical than using a PIN or pattern; I kind of doubt I'd have switched over even if it was included, though I admit it would have been entertaining to try it out a time or two. Ah, well...