My theory, like I've stated in other threads, is that there will eventually be a dashboard interface for widgets that will allow limited multitasking of lightweight apps. I also tend to believe that devs will be able to write these small apps and distribute them through a widget section of the App Store. I believe this is a good place to put an app like Pandora and other notification services, like Facebook, and maybe even AIM. Though I doubt it will start with such extensive support.