One of the major features coming in the Apple iCloud is iTunes Match. You will be able to access all your music you have in iTunes from the iCloud from anywhere on any device for $25/year. Apple has not made entirely clear when they announced the feature that you can also stream music and not just sync it. The iTunes Match beta that developers received yesterday has revealed that you can either stream (256kbps) or download songs from the iCloud.
To have both option is actually awesome. If you want to just listen some music right away you can stream them and do not have to wait. If you know you will be offline for a while you can download a couple playlists on your device.
You can watch two videos below from InsanelyGreatMac (via MacRumors) that walk you through iTunes Match.
Apple is planning to release iCloud and iOS 5 somewhen this fall. It looks like we will see the launch in October.Update:
This is not Streaming says Apple.