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.
You've apparently been able to do this with iDisk for a while, but this is the first time you get backgrounding support and controls.
It's not totally effortless and there are a few limitiations: you need to drag your music files one by one [or whole folders] 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. The app itself is free, but you'll need a MobileMe subscription ($100/year) to use it. There's also the tricky question of legality—music companies aren't generally in favor of streaming music without a license, 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.
UPDATE: Sam's been testing this out, and it works like a charm over Wi-Fi and 3G alike, with "decent" audio quality. [Michael Robertson]