Huh. The iTunes Music Store has some introductory playlists for modern progressive rock that you can buy and download.
It’s a little strange that the lists contain no Spock’s Beard (which the iTMS does have) or Flower Kings (which it doesn’t). It seems like it focuses mainly on progressive metal. They may just be at the mercy of what’s available on the store, but it might also be the result of biases of whoever composed the list. Although to be fair there are a number of band listed with whom I’m unfamiliar, so maybe I shouldn’t talk.