iTunes is a loss-leader that it gets people to buy more iPods, which is a high-margin product. Given that the iPod has roughly 30% of the mp3 player market, I'd say it is working.
My personal favorite spot to buy music online is www.allofmp3.com , which lets you chose the bitrate & encoding method for your music - you can get it in mp3, aac, wma, ogg vorbis, ect. And it is cheap too - only a penny a megabyte. Of course, the site is based in Russia, and is cheap due to some quirks of Russian copyright law, but from the research I have done it seems to be legal for US citizens to buy music from them.
That might not be as smart an idea as it seems. When I got my freebie iTunes from the Pepsi promotion (my family drinks a LOT of Diet Pepsi) I immediately ran Total Recorder while playing the songs on the iTunes application. I could save them as MP3's, and transfer them all over the house on my wireless network. I never bought, and never will buy an iPod, and there are millions like me.
As soon as others figure this out, iTunes becomes a loss, not a loss leader. Online music sales have to make sense from a profit standpoint if that delivery method is to become viable. I just don't see it happening with technology always one step (or more) ahead.