Undersound is a design project I'd certainly like to see implemented: it's a system by which mass transit users can share music with users throughout the underground system via a bluetooth network. It's easier to read the original description than explain.

Interesting ideas behind Undersound include the ability to share and surf the music available from your fellow riders or previous users of a station, creating an emotional link between action, place, people and media. One can imagine the emotional tone of a station's "soundtrack" would change based on the time of day.