You can use, for example, the Firefox add-on Flash Video Downloader to download videos from twitch.tv (etc). Their
website has a web interface for downloading videos, but it doesn't seem to support twitch.tv, but
KeepVid does.
EDIT: For the Flash Video Downloader add-on, it only detects / downloads videos that are playing (or started, anyway; they can be paused). Since the Q&A video is actually split into 3 parts, I would assume you could start the video to download the first section (which does work), then skip forward more the half an hour for the second, and then skip to somewhere in the last 25 minutes to get the end (when I tried that I got 2 different flv files, but neither could be played). When you skip forward in twitch.tv a time indicator gets added to the URL, which I think is what is causing a problem with Flash Video Downloader (it seems to be downloading the second and third videos from the marker, rather than the beginning.
EDIT 2:
Twitch Tools
