iDisk is Apple's online storage service similar to Microsoft Skydrive and Google Docs. "Apple recently pushed a new version of iDisk, buried in the release notes is a significant new functionality which lets Apple devices stream music from its online storage in a useful manner for the first time. When you click the "More" button you see this listed: "Play audio from your iDisk while using another app" feature."
What this means is that for any music files such as MP3 or Apple's AAC files stored in your iDisk account you can play them on your devices while you use your device for other operations. This makes streaming practical from Apple's online storage. This new release brings this capability to iPhone, iTouch and iPad. You can stream music to multiple devices simultaneously.
This's not "iTunes in the cloud" but it's definitely moving the Cupertino company in that direction.
More Info: iTunes