Mountain Lion will, with AirPlay be able to wirelessly broadcast their Mac desktop to Apple TV. This is the same ability as iPad user have for a long time. This is again a way that Apple takes features first used on their mobile devices and implemented them back on to the Mac.
AirPlay allows users to mirror whatever is displayed on their Mac Computer to their HDTV. All they needs it the Apple TV connected. Video streaming with audio will even scale what is on the users Mac appropriately. Apple announced that a more sharp image is possible if a resolution is chosen which is near the 720p out from Apple TV.