Amazon released a really cool new service called the Amazon Cloud Player. All your Amazon MP3 purchases are stored there and you can upload whatever you want. This seems like the first big threat to itunes. Amazon has made it compatible with Android.
It's worth noting that I've been buying a lot of my music through Amazon lately because it's almost always cheaper. However, my impulse buys are alwasy through iTunes because that's where I listen to my music. Amazon downloads their MP3's directly into iTunes, which is what got me buying there in the first place. This is another big step. Probably not enough to get me to scrap iTunes but it's a small start (note, I don't want to scrap iTunes, I just want to listen to my library everywhere, on any device).