Here's what I see in the future:

All iPods (or a similar device) have built-in FM transmitters/receivers.

There is a new option in your Settings menu: Receive. It can be set to ON or OFF.

While OFF, everything is the same as it is now.

While ON, if you aren't currently playing anything on your iPod, you will hear anything being played by the nearest iPod, let's say a radius of like 5 meters.

Imagine if your iPod produced a "pocket" of music around you by default! You and your buddies all have iPods? Set all but one of them to receive and let one of your buddies be the DJ! Imagine walking down the street set to receive and listening for music from people passing by! Or riding the bus across town set to receive, hearing the song that the guy across the way is playing!