The USB audio version of the Mini DisplayPort to HDMI adapter hasn't shipped yet, but Kanex just came out with an even more updated version of it with digital audio.
There isn't a whole lot to explain, other than the fact that the adapter is now basically set to deliver full audio quality to the video content played back on your Mac, and all through one HDMI cable. The previous version was held up because of manufacturing issues, so we'll have to see if this version suffers from the same difficulties. [Kanex]