Frustratingly, Mini DisplayPort doesn't include audio, so to get sound when outputting to an HDTV using HDMI, you'd need to use yet another cable. This adapter uses USB for audio, so you get the full A/V with just one cable.
Though HDMI is capable of handling both audio and video, the Mini DisplayPort on MacBooks is not—a definite source of annoyance for MacBook users. The Kanex adapter gets sound from USB, like one of those teeny external sound cards, and merges it into the video it gets from Mini DisplayPort, so on the other end, your HDTV gets both audio and video from HDMI, up to 1080p resolution. It's a simple solution, though of course not as elegant as if Apple would just put a damn HDMI-out in their MacBooks, so if you're showing a lot of HD video content, this could be a real life-saver.
The clumsily (if descriptively) named "Mini DisplayPort to HDMI adaptor with USB digital audio" from Kanex (a line from Apogee, makers of various high-end A/V products) will be available this August for an undisclosed price. [Press Release]