Maybe I am the last to discover but as I just skipped through some pictures on G+ I recognized some existing boxes that would appear on a photo when you hover your mouse cursor over someones face.
Now tagging photos isn’t new and we all know that you are able to do so on almost any website that features pictures. What worries me (if only just a little bit at the moment) is that nobody put that rectangle there manually which means Google has already implemented some at least very basic functions to recognize people’s faces on pictures. Currently it seems to work only with portraits, views from the side aren’t pre-tagged.
Let’s not hope they will go so much further as to add suggestions who this person might be, like Facebook already does and what may lead to some serious privacy issues if they went for that same opt-out policy.