There are already several research project going in the direction of City interactions, grouping and portable devices. The best contribution are represented by the Geograffiti project and the activity of Antony Townsend. In addition, the JavaGPS gorup is building the same stuff we are planning to do. In this sense, a new dimension into the field may only be added looking for the specific field I targeted, which is the understanding of the cities.
What can really make the difference is to design a service which make usage of this particular cognitive context to add something to human communication.