I've been using the PBS app on the iPad and AirPlay to stream to the AppleTV. This gets a lot of good content, but the expiration date is pretty short and they do insert some ads in some programming. From a cursory look, the AppleTV app doesn't offer the same content, such as NewHour programs less than a day old. Yet another selection of content is available through the PBS mobile web interface. It's nice to have yet another way to view PBS programs, but it's altogether kind of a mess, not knowing where to look for any given program, especially if you're not into endless reruns of Downton Abby.
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The reason why they haven't released a high end device in this space is because the rights issues are so intractable. Everyone who is not a content creator or a cable company wants the system to change. Unfortunately, the content creators and the cable companies control the entire system and they don't see any reason to disturb a model that works so well for them.