Steve Jobs and Apple are making the rules right now, and it looks like they're getting closer to how I hope (and predict) it will play out. They're charging more for unlimited plays, and I see this as a move to controlled music licensing. My prediction:
Songs (and videos) will be sold based on number of plays. Here's an idea of how they could be named and priced.
JukeBox Version: $.25 per song.
Installs on one machine (computer) and one player (iPod, MP3, CD); 5 plays and self-destructs
MixTape Version: $1.40 per song.
Installs on one machine and multiple players; 70 plays and self-destructs
Vinyl Version: $2.00 per song.
Installs on one machine and up to 3 players. Unlimited number of plays.
Groupie Version: $4.00 per song
Installs on one machine and up to 3 players. Unlimited number of plays. Additional feature allows owner to share up to 20 "JukeBox" versions of song with friends.
Update 04-13-07: Maybe a subscription model is coming too. ;c)
Videos would have different pricing, but you get the idea. Steve Jobs, call me.