Bluetooth is well fast enough for a few mp3's
Say your in a restaraunt, do a quick bluetooth search, find a phone with the sharing app and start downloading, just like Kazza you could set up the folder to share.
Of course you may walk out of range halfway though, but it will work fine in places where everyone stays put for a while..trains, buses , movie theatres..etc.
You could share, ringtones, videos, with MMC cards getting bigger all the time, people could have loads of stuff to share on their phones.
Of course cracked apps will appear..but that's life.