I have had my iPad on iOS 5.0.1 for a while, and restored it a couple of months ago, wiping all the content. Today I put some music back on (this is iOS 5.1.1 now) and while the music player works, as soon as I close it the music stops (I say 'stops' rather than 'pauses' because using the play button in the multitasking tray starts the song from the beginning as if the Music app hadn't been running.
This does not happen when playing music from other apps such as Dropbox.
Any ideas?