On topic, a friend of mine whose 4s I jailbroke suddenly would not play any of his iTunes library on the phone.. even though stuff like Pandora etc., still worked fine. The library was downloaded to his phone pre-jailbreak. He had also downloaded some Winterboard app that was not 5.0 compatible, and this began his problem I think, but then his jailbreak also seemed broken and nothing was available for removal or re-install in Cydia.
Long story short, I had to re-install his jailbreak, re-boot the phone, delete all the crap apps he installed but wasn't using, delete his entire music library, and re-install his iTunes library again on the correctly working stable jailbreak.
No problems since...