While Apple offers an online application through which you can ask them to 'consider your music for the iTMS,' Tunecore (notice the hip 'beta' marking) can actually submit your music to not only the iTMS, but Rhapsody, MusicNet and Napster too. They offer what I would consider a reasonable one-time fee structure (meaning: the don't take a % of each song/album sale), and they can add your music to a number of international stores (not just the U.S.) with 'more coming soon!' One last feature to note about their service which might be music to indie musicians' ears: you keep all the rights and ownership of your music, i.e. - Tunecore a middle-man who actually remains a middle-man.
This sounds like a great service. I hope more musicians can use it to hop on the digital distribution bandwagon.
[thanks Andre Dupont!]