I suppose you could use machine learning to first identify human faces and then compare facial features against other features in a data set. Human facial recognition software is starting to get pretty good, and it's already being used on facebook.
MyHeritage.com seems to have a "celebrity lookalike" facial comparison application, as well. But it requires a free signup: http://www.myheritage.com/FP/Company/tryFaceRecognition.php
