9to5 Mac has found references to face detection APIs in the current developer build of iOS 5. The iOS 5-exclusive APIs include CIFaceFeature for identifying a the mouth and eyes in an image of a face. Additionally, CIDetector is a resource within the OS meant to process images for the purposes of face detection. Further iOS spelunking turned up hasLeftEyePosition, hasRightEyePosition, hasMouthPosition, leftEyePosition, rightEyePosition and mouthPosition.
Either Apple is building a Cylon Resurrection Ship or iOS 5 will offer some cool face detection options to developers. In either case, it seems Apple's recent purchase of Polar Rose, which specialized in face recognition, is about to pay off.
Update: Face detection was announced as part of iOS 5. It's a fully documented API that developers can use immediately when iOS 5 launches. That is to say, it's not a surprise, at least to some developers.