It's an obvious counterpart to Nokia's 'Comes With Music,' and follows the model of opening on a region by region basis (and that's probably more to do with the music publishers than a conscious choice of strategy). There's no indication of an on-device client just yet, but you have to assume one will be on the way.
The mechanics have the story working on a 'per-track' purchase of roughly 9 Swedish kroner per track, rather than the Comes with Music all you can listen to policy that Nokia is implementing - although it should be noted that Nokia's Music Store is currently operating in the same purchase per-track mode right now. Finally, Sony Ericsson's Martin Blomkvist noted that after they had paid out the labels, the network operators and the taxmen they would "hardly profit." Which says a lot about the music industry, but also that Sony Ericsson feel the need to launch a music store in the current environment, even if it's barely a revenue stream.
More at The Washington Post and Sony Ericsso's Press Release.