Shotwell is going to have a face recognition tool and the developer, Valentin Barros, posted some nice insights of how it would be like.
This tool will detect faces in your photos using the OpenCV FaceRecognizer Class, and store them in a database. Later when when you open any photo and press the detect faces button, the tool will scan the photo for any faces and ask questions to the user like is this Mr X? If its the correct person, the user can press enter to save the tag, else user will be given an option to enter the correct name.
This tool has the success rate of detecting faces by 30% accuracy on average, but this can be drastically improved to 80% by merely adding and tagging more pictures. Though this is enough for normal use, Barros is still working out a way to increase the accuracy further.
We are waiting for screenshots and more than that, we are waiting for Face Detection to ship with Shotwell. It is a smart tool that all people will definitely like in next Gnome releases.