RIP Napster -- for the time being...
Jeff Kandt has suggested embedding XML markup in the ID3 tag of an MP3 to provide a voluntary "goodwill" payment mechanism for MP3 spreading.
This is an interesting proposal, and could work, although there's 2 problems I see:
IIRC there's a problem with using credit cards for small payments like this, so we're back with the micropayment problem again.
Also I think it would need some kind of better authentication to protect against someone stripping out the XML (sig and all) and replacing it with their own, hacked or otherwise dodgy, server information. Maybe a widely-distributed list of valid servers, or a set of well-known trusted servers you could visit to verify that the link between MP3 and payee is valid.
Still, good proposal.