The unspoken dark side of the iPod phenomenon is that music you buy from iTunes can only be played with iTunes or an iPod.
Some people think that Zune, Microsoft's coming MP3 player, is a strong contender, that Microsoft can challenge Apple's hegemony in a way that Creative can't. I wouldn't take that bet. Anyone who has bought more than a few songs from Apple is tied in. Those songs will not play on any other player. You can change them to MP3 format and use them on other players but most people won't understand that.
Apple was smart. Take the lead, build the brand, establish the presence -- and technically create a barrier to entry.
It's Microsoft's third biggest problem. (Their second biggest problem is open-source software. Their biggest problem is Vista.)