It seems that if you're a band, and you can get your song in an Apple iPod commercial, you can expect a surprising amount of success.
The most striking example of this is when Nick Haley, an 18-year old university student in England, made an amateur commercial for the iPod Touch. He paired his images with a song named "Music is My Hot, Hot Sex" by a Brazilian band called CSS (short for Cansei de Ser Sexy which translates to "tired of being sexy"). The commercial impressed Apple and they brought Mr. Haley to Los Angeles to to professionally make his commercial and they kept the same song.
Since the ad debuted, the critically acclaimed band, which had no commercial success in the USA, has climbed the charts, selling thousands of albums. Feist has also seen similar success with their song "1234" being used recently in ads for the iPod Nano, and has since sold tens of thousands of albums and landed a spot on Saturday Night Live.
There has been no mention of how Apple choses their songs, but it is sure bands are going to try to crack that mystery.