A team in Gothenberg, Sweden, has created Push, a WiFi-based music-sharing architecture that automatically connects any two (or more) Push-enabled MP3 players that come within 20 meters of one other. The devices then use simple music profiles to decide which tracks to trade.
For this to work ethically, of course, there'd need to be a hacker-proof Digital Rights Management (DRM) regime that allowed recipients of new tracks to sample and then pay appropriate royalties--let's assume, for the sake of discussion, that such a regime can (perhaps through divine intervention) be made to work.
Push is interesting partly because it has the same basic transmission requirement as infectious disease: proximity. Hence it could be an illuminating way of studying deadly pathogen propagation: How long would it take a "fatal" piece of hip-hop (granted, they're all fatal in my opinion) released in Manhattan to make its way to, for example, Minsk? (This may be a moot question--what with air transportation, the answer from any town to any other town is probably "less than a day." Be afraid.)
Push is also interesting because it's passive. You don't seek tracks; they seek you. So, just by virtue of walking around, you're getting an erratic, eclectic stream of files--as a way of exposing yourself to new music, it isn't a bad technique. I also like the fact that you could collect a voluminous track database just by dangling your MP3 player out your apartment window for a week or two (think of it as angling for bits), with your eventual mix depending on what part of town you live in.
Granted, you could get a similar effect with centralized architectures, but Push has the advantage of being cool--also, of course, Push traffic is impossible to monitor (it's truly peer-to-peer, rather than peer-through-intermediaries-to-peer), so in the absence of strong DRM, music piracy services could use it with impunity. All in all, a very provocative piece of work. I hope we'll see it (or something like it) built into players in the near future, and that your player and my player will be able to exchange songs some time shortly after that. And who knows? Perhaps you won't hate Tom Waits as much as my wife does.