The Lucid specifications for the Ubuntu One Music Store on the Ubuntu Wiki mention "The Lucid music store project aims to deliver the ability to purchase music from within a desktop music player. The overriding requirement of the project is to minimize engineering effort. This will be achieved through the usage of off the shelf solutions with minimal modifications. Only changes classified as blockers will fall within scope of this project. No effort will be expended towards improving the user experience of the default off the shelf components."
The first package that provides a plug-in to Rhythmbox for accessing the Ubuntu One Music Store has been pushed and can be found in the Lucid repository. This is the first package for the Ubuntu One Music Store that has been developed by Canonical.
When looking at the source-code to this Rhythmbox plug-in, it appears to be incomplete and does not appear to actually hook into any actual store. It's believed though that the Ubuntu One Music Store will end up running off of Amazon's MP3 store, which is quite nice and already available on Linux. It is clear though from this code that the audio formats will be MP3 and not Ogg or any other free codecs, which is sure to upset some users.
It's expected that plug-ins will arrive for other media players as well to support the Ubuntu One Music Store. However, all music downloaded by the plug-in is stored within ~/Music/ for easy access by other media applications.
Look for more on Canonical's Ubuntu One Music Store as the Ubuntu 10.04 LTS release approaches in April.