Apple's latest update to its iDisk app, on top of some other upgrades, allows music streaming for the first time. It isn't quite the full iTunes sync like some people want, but it will allow you to stream any music that you have stored on your iDisk to any devices that have the software. It even allows for background streaming, which is quite nice for those on the move.
There are some drawbacks, such as the lack of music auto syncing and the lack of playlists and album artwork on the service, but if you are just looking for a way to get a ton of music in the cloud, this is great.
The funny thing is that this music streaming feature is not included in the bullet list of added features that shows up on the main screen, it is almost like Apple is hiding the functionality. One reason may be that "Apple's service allows unlimited sharing (no username or password required) and now background streaming - all without a license from the record labels" which is apparently a big no-no in the industry.
Regardless, you can pick up your free update to iDisk today. Have fun!