I stopped by a demo by Sean Savage this morning on PlaceSite, a system for letting people share information (like who they are, where they are, etc.) locally over Wi-Fi hotspots. Say you're in a coffee shop in downtown Salt Lake and it has a PlaceSite installed, when you open up your browser, you see not only who's there, but even who's close-by. It seems well designed and it's open source. Sean and two of his friends (Damon McCormick and Jon Snydal) built it as a part of a MS project at Berkeley this year.
Scott Lemon and I have discussed this kind of idea before for community service announcements, local advertising, etc. Here's a platform you build on Scott.