Preview at WWDC likely in June, followed by September launch.
Video Walkthrough of AirPlay and Other Features in OS X Mountain Lion
InsanelyGreatMac has put together this nice walkthrough video of OS X AirPlay mirroring. That feature allows you to transmit your OS X desktop onto your Apple TV over Wi-Fi:
Meanwhile, CultofMac does a walkthrough of 30 other features in OS X 10.8. Some previously listed, and some new:
There's more discussion on OS X Mountain Lion in our forums.