The term "presence" makes a lot more sense to me now I know it came from the shortened "telepresence".
Instant Messaging systems have a form of presence built-in.
The good news: They've already written it, and Glancing uses it.
The bad news: The model it follows is the most simplistic view of the real world. It's completely synchronous with time. A person is there, or not, corresponding with whether they're at their computer. But in the real world, the scent of a person lingers in a room, or they leave their stuff scattered round the office. Anonymously, people leave footprints in the flow of the environment (extelligence, using the word the ubicomp guys use). IM models this a little with the "Away" status, but it's pretty half-arsed.