Joel asked a question in my comments here.
It's tough to say what will happen now that Real Networks and Microsoft have teamed up. What usually happens when two mediocre players get together to try and make something great...
My prediction is that nothing interesting will happen. Microsoft doesn't have the whole platform, and neither does Real Networks. I don't think real has been a significant player in the online space for many years. (At least they haven't been in my conciousness for a long time)
Apple is definitely focused on the music platform. It will take something radical to blast Apple out of it's position as market leader. I think Apple has positioned the itunes music store very well. The store is designed to support the iPod. An iPod is something tangible, sure music has higher profit margins because the distribution costs are next to nothing.
I would predict that in order to replace the iTunes conglomerate there would need to be something that really addresses the rub of online music distribution.
I wont buy songs through the itunes music store because I am more restricted in how I can use it. Since Apple's DRM limits what I can do with the music, it is less valuable to me. I recently bought a CD at Sonic Boom that cost 17 bucks. I could've gotten the same music from iTunes for about 12 bucks. The extra five dollars were worth it to me though because I could put that music on both Gwen and my laptops, as well as have a hard copy in case those digital files are lost.
What does that mean for iTMS? How many people are not buying music because of that? Hard to say really. Why wont I encode my music as AAC or WMA? That one is not so hard. MP3 is a de-facto standard. Every portable music player supports it. Choosing another media format does nothing but benefit the owner of that format. By choosing WMA, I give Microsoft more business. By choosing AAC, I give Apple more business (the business I'm talking about is codec licensing, if I have a ton of WMA I'm going to demand that my audio player plays WMA, which costs the hardware manufacturer to license).
Ok so what does all this mean... I guess it means it's friday and I'm rambling. But it also means that your music player doesn't matter, your audio format does. Choose the platform with the widest support to encode your music in, and then pick the player that fits your model of how you work with music. I choose itunes to play my music because I have an iPod and an Airport Express. If another equally awesome or better set of peripherals come out for Windows Media Player, I can simply pick up my library and move it over to that player.
Now is the time where I would normally proof-read my post... but instead I'm going to not.