Oh, does anyone know if 10.8 has airplay video support in for example QuickTime Player? In such a case the sending device only has to wirelessly transmit the H.264 video without transcoding, like with iPhone and iPad models that don't support the mirror feature.

Or support for images would be nice too, allowing one to run a slideshow. I don't actually often need full mirroring. The iPhone/iPad can do this, but getting photo content from a computer onto those devices is next to impossible in the first place.