Insanely Great Mac has had the chance to demo the new Airplay Mirroring in OS X 10.8 Mountain Lion. From the video, it looks fairly smooth and this will definitely be a big hit when the new OS come out in the summer.
If you are lucky enough to have a copy of Mountain Lion beta installed, chances are you still won’t be able to do this as you will need to install Apple TV Software 5.1 beta 2, which hasn’t yet been released.