The feature came via a July 6th update to the application but has gone largely unnoticed.
This is not "iTunes in the cloud" but it is definitely moving the Cupertino company in that direction. First off there is no automated way to get all your iTunes music to your iDisk account. To load files to iDisk you have to select individual files and upload them from your browser. (Apple does let you sync Calendars, Contacts, Bookmarks, etc directly from OSX but excludes music files.) Secondly there's no support for playlists so your iTunes playlist do not work in iDisk. There seems to be no way to play a list of files. Cover art is not supported as well. And while iDisk will cache other files, it will not cache music files. Still it's not hard to see how Apple is adding features to enable it to support audio in it's cloud storage business.
Robertson notes that this feature is likely offered without the okay from Universal Music Group who told net companies that streaming music files requires entering into a license and paying a per stream fee. Apple's service allows unlimited sharing (no username or password required) and now background streaming - all without a license from the record labels.
Read More [via Marty]