

I've wondered about this - apparently it's also possible to use it as a one-off.
So you subscribe once and it goes off, looks at what you've got (dodgy low-res MP3s included) and then all your songs are added to the cloud in a better resolution.
The you move your iTunes library (to another hard disk, say) and then you can download all the nice new files and you don't need to subscribe again after the first year.
I must say I haven't done it myself but it seems reasonable.