streaming a full screen app or a screen space on apple TV I would like to see if there is a possibility of streaming a full screen app or a space onto Apple TV with MacOS mountain lion. The implication of this is that I could stream for example a tech talk on to my TV while I browse on another screen space. I believe that mirroring would just show the stuff that I see on my screen.
Unless a hack will be  possible or Apple will have an AirPlay API for someone to make an application that makes the Apple TV a 2nd display it does not look possible with the advertised OS X Mountain Lion feature AirPlay Mirroring, since it will only allow mirroring of what is on your screen, probably only your primary screen too.

Wireless mirroring
Send what’s on your Mac screen to an HDTV wirelessly with Apple TV. Mirroring is great for classrooms, conference rooms, and your living room.