http://www.we-make-money-not-art.com/archives/003798.php

Mattias Östergren from the Interactive Institute in Stockholm has developed SoundPryer, a wireless peer-to-peer software for mobile music «socials» in cars. Each driver becomes a mobile radio station, transmitting their digital music stream to other cars within Wi-Fi range. It enables music eavesdropping in traffic encounters, by streaming MP3 files via the Real Time Protocol.
How it works: you attach a PDA, with built-in Wi-Fi card, to the dashboard and starts the Sound Pryer application and play Chopin on the car stereo. Suddenly, the icon of, for example, a red lorry appears on the screen and a tune from J-Lo fills your loudspeakers. Sound Pryer returns to playing your own music after you’ve passed the lorry.
More details in the PDF.