You can't "scan" the content of video comments like you can, say, this comment. It's an interesting idea, but seems to disrupt the flow of communication.
Case in point, I just watched a 20 second clip of Arrington saying, "Um.. Uhh.." and not much more. His text comments are obviously a tad more engaging.
These seem much more suited to a video site, like YouTube, than a comment thread. Please, Hacker News, don't adopt video comments!
Doing video polls sounds more interesting, with the poll-reading community up-voting the best video comments for each given question.
Besides, I only watch videos from the 70s. If you want me to watch your video comment you'll have to travel back in time and appear on Don Kirshner's Rock Concert or The Old Grey Whistle Test.