OpenCV 2.4 now comes with the very new FaceRecognizer class for face recognition. This documentation is going to explain you the API in detail and it will give you a lot of help to get started (full source code examples). Face Recognition with OpenCV is the definite guide to the new FaceRecognizer. There’s also a tutorial on gender classification, a tutorial for face recognition in videos and it’s shown how to load & save your results.
These documents are the help I have wished for, when I was working myself into face recognition. I hope you also think the new FaceRecognizer is a useful addition to OpenCV.