[the iPod confers] incredible levels of control over what we listened to at any moment. It’s simply next in the progression from LP (moving the needle from track to track), to cassette (pressing FF and guessing), to CD (pressing next, but still limited to one album). Now, at your fingertips, there is the power to pick any song, play it for any length of time, and skip to another song, and keep skipping until you find what it is you want to listen to. While there is great, great joy to be had in simply shuffling at random (the wild success of the iPod Shuffle definitely illustrates this), I think all will agree that it is not enough.
Well, it seems simple enough : equip your player with a Bluetooth transceiver and hope enough other player owners do too. Tuck a protocol between the Bluetooth and your player, such that when two players come within range of one another, they compare their inventories and exchange songs that the other does not have. For intellectual property purists, set a flag to play it once and delete it, but leave a chit identifying the incoming, once-played track. At the end, sound a prompt to the listener, "keep, or clear?" If you choose to keep it, it stores the artist, album, label, license terms and so forth so you can find it and (if the artist demands) buy it.
If you are among several people with shareplayers (a long workout, say), enter a share-ffle (shruffle?) mode that gives each player a turn to offer a track to everyone else.
A lurker mode allows you to walk down a crowded street, allowing other shareplayers' tracks to fade in and out of hearing. The protocol already skips tracks you already have, could even be told to skip artists, albums, labels, and genres you choose. FF allows you to skip a track from one player and go to the next-most-audible player. Play grabs a track, or its tag, whenever it sounds interesting.
As much as it pains me to say it, mobile phones with mp3 features are probably better-positioned to deploy this capability than straight mp3 players. They already pack Bluetooth and they're supported by companies that have a history of charging out the ass for simple features (remember paying a separate fee for "tone dialing" on your landline phone?).
As always, this reminds me of High Plains Drifter: somebody is leaving the door open and the wrong dog can come home. Serious consequences for a mobile phone. Equally serious for a non-phone mp3 player that eventually comes to dock in at a computer. But these are the same risks that file sharers take already.