Ok, I'm pretty sure this was not possible on Mountain Lion and I think it's probably enough reason for more than one to upgrade to Mavericks. The thing is, with Mountain Lion I had to keep my MBA lid open for the streaming to work. If the lid was closed, the playback would stop and the computer's library would't appear on the AppleTV. Now, with Mavericks the computer's library can be loaded and movies and other stuff (including photos) can be streamed even when the computer is sleeping and with the lid closed. I think this is a great addition, I had been wishing for something like this to happen for a long time. I also don't think this has been posted anywhere. And, if this was possible before Mavericks please excuse me, I found out today by accident. But I can tell you that I'm pretty happy about this For the sake of it, my MBA is a Mid-2012 and the Apple TV is a 3rd Gen and I do the streaming wireless through an Airport Express.