Well this was unexpected: Apple has quietly enabled its devices to stream music to iPhones, iPads, and iPod Touches from online storage through iDisk. It's the very next-best thing to putting iTunes in the cloud. UPDATED:
The new feature was unearthed by Michael Robertson, who also provided the screen shots above showing clearly that iDisk allows both background music streaming and public file sharing among multiple devices at simultaneously. Matt declared iDisk "halfway there" in his lightning review—this seems like as good a second half as any.