Direct file transfer between portable devices

Sharing files between two PCs is easy: you can do it over a home wifi/wired network; you can do it over a direct cable connection network; you can do it over a ad hoc wireless network; you can transfer files with a memory stick; or you can use the internet. Sharing files between portable devices (e.g. mp3 players) currently requires a computer. Would it be possible to create a portable device (no larger than an mp3 player) to all direct sharing of files between portable devices? Or is this perhaps a function for a smartphone? Perhaps you could plug the mp3 player into the smartphone to transfer files too or from it?