Cinematize is a very useful program for that: I used it to recapture a video stream from a DVD that I had authored in iDVD v.1, without recompressing or decoding the stream, so that it had the same visual quality, which I couldn't manage to recreate from the QuickTime files (actually, I think I'd nuked the QT after making the 1st video, thinking it wasn't THE original source file.) Cinematize is about $79 or $89, and well worth it. I heard about it here, from one of the wonderful info sources who come online.