Boxes and Arrows more and more deal with locative media. They have a paper by Jonathan Grubb and Shawn Smith called "Location and Presence in Mobile Data Services". The authors advocate for the importance of presence awareness (added to location-awareness feature). They then propose ideas for "designing for presence":

When designing presence into applications, designers need to understand the two basic types of presence: system-generated status and user-set status. “Online” and “idle” are examples of system-generated status. Location is another. “Busy,” “bored,” and “invisible” are examples of user-set status. System-generated status is important because it ‘s accurate, reliable, and objective, but it risks revealing things the user does not want to reveal. Therefore, the system, should give the user the ability to suspend, change, or override system-generated status unless there’s a compelling reason not to do so.

In general, systems should not use presence data to set limits that second-guess the user. Rather, systems should use the data to enable the user to make informed choices. For example, even if a user has set her status to “EXTREMELY BUSY,” the system should not prevent someone from calling her.

Designing for presence in general makes for an interesting discussion, but presence has especially profound implications for the design of mobile data services.