From the google blog:
Until now, you could only get FLV files from your browser’s cache or using one of the many websites that let you download YouTube videos. In fact, to download the MP4 files, you need to use the same URL like for FLV files and append “&fmt=18”:
http://www.youtube.com/get_video?video_id=ID&t=SIGNATURE&fmt=18
ID is the video’s identification value, SIGNATURE is a value that prevents you from downloading the file just by knowing the ID.
More here:
http://googlesystem.blogspot.com/2008/04/download-youtube-videos-as-mp4-files.html