RecognizeMe comes from the developers of 3DBoard, the interactive 3D homescreen on iPhone, and is available in Cydia market for $6.99 via BigBoss repo. So if you feel you are too lazy to type in the password to unlock your iPHone, or just wish to try out the fun, its worth buy. The app can be used with all devices having the front camera: iPhone 4, iPod Touch 4G, iPad 2 (when jailbreak is out).
RecognizeMe is the first of its kind facial recognition on iPHone, and its not perfect, but does a decent job identifying you. If you are looking to replace the passcode with facial recognition, you might find it slow and less intuitive. However, the app can be trained via settings to recognize different kinds of facial perspectives.