This service will be operative by the beginning of June. Apple has taken seriously its preparation and has achieved agreements with 3 of 4 most important multinational Music (Warner, EMI and Sony). Universal Music is still pending, but they are still in discussions.
ICloud will proceed as follows: it will scan the content of your library on your computer (and mobile) and store it in your own virtual data of songs (not the music, only the title, artist, album, etc..) and then, when you want, you will be able to access to that content through on-line connection from the device associated with the account you want, whether physical or mobile. So we will not have to fill out our mobile phone with songs from the computer or vice versa.
As wells, it seems that you will be able to share your library through Facebook and, if they like, they will be able to purchase through iTunes Music Store.
As I have advanced, it is still pending to be confirmed but rumours would be almost exact. It will be definitive in the coming days.
More info | Apple