I was just thinking about iTunes this morning as I was downloading the new Big Boi record. iTunes is the only option to download music on my iPhone. I'd rather download music from Amazon mostly because they offer their catalog in mp3 rather than AAC format, because their albums tend to be a dollar cheaper, but mostly I think Apple could use some competition.
Amazon has the MOST outstanding iPhone app I've ever seen. It's not quite as easy as the website for doing heavy research, but only because of the size of the screen. I actually think the interface is better on the iPhone app. All the information you need is right there, and it's as intuitively laid out as it could possibly be. You can track your order, the search is fantastic and all of the product reviews are right there. Unfortunately, due to the rules in the iPhone SDK and the restrictions that Apple places on iPhone developers, you can't download music from Amazon to your iPhone. There's nothing preventing this feature from a technical standpoint, though Apple would argue that outside developers might write crappy software that would degrade the performance of blah blah blah. I don't think Amazon writes crappy software. The only real reason is to prevent competition.
iTunes is the "largest seller of music on the planet". I'm not in the mood to do any research on the market share of the iPod and the iPhone in terms of smartphones and mp3 players, but it's obviously substantial. In terms of mobile delivery of digital music, it's a virtual monopoly.
If/when the DOJ (or more likely the EU) starts making noise about opening anti-trust charges against Apple to force it open it's platform up a little bit, I'll be cheering them on.