I jailbroke just so I could run Pandora and Slacker in the background. (Though an updating weather icon and having a background image are nice too.)

I'm not sure how Pocket Tunes Radio is able to play in the background, but they've found some Apple-sanctioned solution that works for MP3 and AAC+ streams. Or I can use Safari to start an MP3 stream and it will background just fine.

I'm sure Apple could just make a policy decision to let some apps they approve run in the background.