BananaScreen is a unique, freeware application for Windows that allows users to lock, unlock and login to their system based on facial recognition technology using a webcam. When you step away from the computer screen, BananaScreen locks the computer automatically. And when you come back, BananaScreen unlocks the computer using facial recognition, taking less time than it would to enter a password.

From the site FAQ,

Facial features are captured to build a statistical representation of the face. No picture is stored on your harddrive (or anywhere else) but only your face model (binary format).

Download BananaScreen here.
A Mac OSX version is under development.