You know, one of the cool things about Apple Safari (besides that it's free) is that if you're watching a youtube video and append &fmt=18 to the address then go to Window>Activity (not sure where it is in Safari 4 on Windows) and then expand on the name of your tab, you'll find a file that is several MB long. If you click on that file, then press Command+C (Ctrl+C) then paste it in the address bar, it will download the MP4 file.
...Of course if you don't add the &fmt=18, it will download an flv.