I just realized we could take one of those security doors, hook it up to a computer, and have that computer put the information out in an RSS feed. If we had cards or numbers specific to individuals, then we could set it up so that we would know who went through the door. It would be similar to Twitter, but site specific.
Now, a door implies a room, I hope, so if I'm in this room, folks could find out, and if they wanted to, they could come hang out. Unstructured!
Man, I wish I owned a restaurant! This would be a great experiment to do with a private room in a restaurant or coffee shop. We could see if more interaction would happen simply by having a location (with good ambience), and real-time knowledge.