The idea is to stream your Ubuntu One Music folder/library to cellphones via an application and over the web to non-Ubuntu machines. Since the user already owns the tracks offering them for mobile streaming should cost Ubuntu One nothing besides the implementation (note the *should*, record labels rarely agree with what *should* be possible).
The website could even have ads-By ads I mean internal ads to music/bands you can buy via. the Music Store. Every little bit of profit helps =D
Intensives for Ubuntu One subscribers- a radio service like Pandora or lastfm that offers tracks that have not been bought for streaming. Thus adding value to the subscriber plan and encouraging the discovery of new artists--> more sales. A free version of the above could be offered for Indy artists that opt-in or as part of a promotion.
"Sync your streams/playlists"- I am not sure how much demand there is for this. But as an expansion of the streaming service for subscribers you could offer to sync music streams (ie. so two or more people are listening to the same thing). Kinda like stations that other services offer but for small groups. For example if two people are chatting on Empathy and one user really likes a track he/she could share his/her stream of playing tracks with friends. (There would have to be limits here including limiting this service to subscribers because record labels would demand compensation for this). Moreover, a link could be offered via Empathy or the music player to "buy it now".
Offer the "Music Store" via the web/mobile-application- Hear something nice on the radio in the car? You can buy it now but don't have to download it/waste your data plan till you get home. Moreover, streaming users could be offered a direct link to "buy it now" via the mobile app.