If Apple, Spotify, Napster and their ilk think they are going to have the mobile music business to themselves in the coming years, someone had better point them towards the newly-launched Rdio. It just might ruffle a few feathers.According to Venturebeat, Rdio (pronounced r-dee-o) is backed by Skype founders Janus Friis and Niklas Zennstrom, who seem to know a thing or two about net-based businesses, although this music service is as much about your mobile handset as your computer.Initially launching in the US as an inviation-only beta, you can subscribe for $4.99 a month for an unlimited number of songs from a database of over 5 million songs from the major labels. But more interestingly for us, a $9.99 a month subscription will let you listen to music on mobile phones - there are iPhone and BlackBerry apps already available to use, with an Android app soon to follow, all with access to 'cloud-based' music wherever you are.Happy with your iTunes collection? Well, Rdio has a clever little trick up its virtual sleeve. When you sign up, Rdio can compare your iTunes collection to what it has in its own cloud-based music collection, offering up those very same tunes to playback anywhere. Nice. It's also a social music service, with the option to listen to music purchased by friends. There's also Facebook and Twitter integration.'Many digital music services have tried to address specific challenges, but no one has managed to elegantly integrate discovery and a high-quality music streaming experience under one roof,' said Friis. 'We think people are ready for the next evolution in music. Rdio gives you unlimited access to all the music, anytime, for the equivalent price of one album download per month.'So, when will it hit the UK? We can't answer that, but with the service still in beta in the US and with UK music licensing laws to tackle, it could be some time. Saying that, we suspect Rdio will make it over here in some form. It's very much a logical next step for digital music.