Google has silently added face recognition abilities to its image search. It is now possible to search for images that only contain faces by appending the "&imgtype=face" query string onto the end of a search URL. There is currently no way to indicate that you only want to search for faces through the image search interface. As well, adding on "&imgtype=news" displays only images that are associated with news stories.
The technology appears to be the fruit of Google's 2006 acquisition of Neven Vision, a company that had developed techniques for facial recognition in photos. Google is apparently taking that technology to heart by experimenting with facial recognition online. Even cooler (or creepier, as the case may be), one day Google's image search may be able to find faces of specific people based on image analysis/recognition alone instead of relying on the text associated with that image to identify the person in the photo.