The (alleged) strategy by Apple that Tim Windsor mentions, if true, is very interesting. The model sounds similar to those of Streamload and the original mp3.com, but Apple may be able to pull it off. If does take off, there is no reason why others couldn't adopt similar models--i.e. different ways of allowing users to play the content they 'own' over the net. Nothing says that these services need to be offered by the same people who sell content to consumers (any lawyers out there who could confirm this?)
If I can prove to you that I own a piece of content (i.e. that I have a perpetual licence to play it back) then you could offer to play it to me at my leisure for a small fee, or in exchange for a bit of attention. I might even appreciate a bit of advertising: if for every 20 songs I play from my collection you add one that I like, we both win: you might get to sell me some content (I might want to 'own' what I just heard, so I can play it whenever I want), and I get to learn about new music.
There's only one snag: how do I prove to you (so that you can prove to content owners) that I own all the content I claim to have? Maybe I could ship you all my CDs, but that is cumbersome and few people will do it. If satisfactory ways of doing this are developed, then legally certifiable lists of content owned by consumers will become an essential commodity. If no such means are developed, these lists will be strategic differentiators. Thanks to iTunes, Apple will have some good lists. They are by no means exhaustive, but they are a good start.