I would really just keep it simple for now, by providing an easy way for the users to buy an application straight from the AppManager, by paying for it with credentials entered in the configuration (or on the first payment). Let us forget the DRM, the mobile payments, and other bells and whistles for now. They are not essential for the first stage. What is important is the first proof that Maemo can be a viable platform for commercial applications, even if there is only a bunch of them at the first stage.