I really like the song that Apple is playing in their new Macbook Air commercials. (Plus I think the laptop itself is pretty sexy.) I'm currently using Rhapsody To Go as my music service because it works with Pocket Tunes. So I searched for the song (it's "New Soul" by Yael Naim) on Rhapsody.
Sure enough, it's not there. You have to wonder if Apple purposefully chose a song that was only available through iTunes.
Luckily, though it's available as an iTunes Plus track, meaning it's unprotected. So it plays in Pocket Tunes.