I\'ve started work on an application for iOS that would recognize faces from a photo or from the iPhone / iPad camera.
Existing solutions like OpenCV and Core Image
On iOS 5 you can use CoreImage (CIDetector, CIFeature, CIFaceFeature should be named as the relevant keywords) for that task. Check out the SquareCam example App from Apple, it includes face detection. If you're targeting older iOS versions, openCV seems to be a good approach.
http://developer.apple.com/library/ios/#samplecode/SquareCam/Introduction/Intro.html
Edit_: Argh, soory. CoreImage can only detect faces but not recognize them. But maybe you can build a solution based on CoreImage...