The Familiar Stranger: Anxiety, Comfort, and Play in Public Places from Eric Paulos and Elizabeth Goodman (Intel Research):

The Familiar Stranger is a social phenomenon first addressed by the psychologist Stanley Milgram in his 1972 essay on the subject [1]. Familiar Strangers are individuals that we regularly observe but do not interact with. By definition a Familiar Stranger (1) must be observed, (2) repeatedly, and (3) without any interaction. The claim is that the relationship we have with these Familiar Strangers is indeed a real relationship in which both parties agree to mutually ignore each other, without any implications of hostility. A good example is a person that one sees on the subway every morning. If that person fails to appear, we notice. (...) There is a special class of Familiar Strangers called the socio-metric stars. These are individuals who stand out in a community or group and are readily recognized by an extremely high percentage of people.

mobilecommunitydesign.com wrote a nice summary of the paper:

The neat thing about this study is that it shows how we move, and how relationships exist between us and other people that we don't consciously recognize. We are part of groups we don't know about, and those groups have the potential to become communities. Although some of the scenarios are a bit doubtful, it's a great example of personalizing a place to your own needs and learning more about your environment and those around you via technologies you control.