Basically using a WiFi card and a PDA playing MP3s you can share your music with others who drive past you in the car. By the same token, when somone drives past you, you get a blast of their music. So you can finally work out what that gorgeous girl or guy is bopping their head too.
I like ideas like this that answer a common social question and help to reinforce the sense of the human dynamic and connection between us all. It’s rather like a musical equivalent of the Blogosphere’s trackbacks.