Well this was unexpected: Apple has quietly enabled its devices to stream music to iPhones, iPads and iPods touch from online storage through iDisk. It's the very next best thing to putting iTunes in the cloud.
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. Matt declared iDisk "halfway there" in his lightning review - this seems like as good a second half as any.
It's not totally effortless and there are a few limitations: you need to drag your music files one by one to iDisk, there's no playlist support and you're limited to 20GB of storage. That's still a whole lot of songs, but some users may not be able to fit their entire music library. There's also the tricky question of legality - music companies aren't generally in favour of streaming music without a licence, something that may be holding up iTunes Live.
But that's Apple's problem, not yours. The only thing you need to worry about for now is which tracks are hitting your iDisk first and where you want to play them from. [Michael Robertson]
UPDATE: Sam's been testing this out, and it works like a charm over Wi-Fi and 3G alike, with "decent" audio quality.