USB is swell. Firewire is swell. That said, portable audio players should have:
- An ethernet connection (wireless is fine, but I prefer wires, or a modular choice)
- A web server
- Some simple remote file transfer mechanism, be it SMB, SCP, SFTP, FTP
This web server/file transfer combination transforms any portable media player into a jukebox for a stereo.
Maybe it just makes sense to sell a hardware device that plugs into an iPod/iRiver USB port that facilitates file transfer and unit control.