All we know about AirPlay right now is that it is able to stream the following ways:
- Music from iTunes to the Apple TV
- Music, Videos, Photo Slideshows on iOS devices from official Apple apps to the Apple TV
Anything else is speculation. Nobody knows if Apple will allow third party developers (like Pandora) to use AirPlay on iOS. Based on the demos it definitely does not seem possible unless Apple releases it as part of the SDK.
As far as streaming anything else but iTunes to the Apple TV using AirPlay, it is pretty unclear. One thing is clear, even without AirPlay you can stream both Audio and Video files that are in iTunes in a format supported by the Apple TV. So if Amazon downloads can playback in iTunes they might play back on the Apple TV.
People might come up with ways of hacking AirPlay to stream whatever they want, but that remains to be seen since it isn't even out yet. So again, that is just more speculation.
Ultimately if you are doing something outside of an official Apple app there is little chance of AirPlay working. For audio I am sure AirFoil will continue to work for streaming audio on your computer to the Apple TV from third party apps.