It's actually pretty straightforward to detect a face; most mid-range digital cameras can do it nowadays. The problem is that even if you tell the program that face A is not face B, all you've done is given it a very small hint in a VERY big problem space. The human brain is hard-wired to detect faces (newborn babies prefer to look at pictures of human faces rather than colorful shapes), which I think makes it feel like an easier problem than it actually is.

That being said, don't let me stop you from trying. It would definitely be a cool (and very saleable) bit of tech.