Here's how i do it:
First, add this to your browser favorites:
javascript:if%20(document.getElementById('download-youtube-video')==null%20&&%20!!(document.location.href.match(
http:\/\[a-zA-Z\.]*youtube\.com\/watch/)))%20{var%20yt_mp4_path='http://www.youtube.com/get_video?fmt=18&video_id='+swfArgs['video_id']+'&t='+swfArgs['t'];%20var%20div_embed=document.getElementById('watch-embed-div');div_embed.innerHTML=div_embed.innerHTML+'<br%20/>%20<span%20id=\'download-youtube-video\'><a%20href=\''+yt_mp4_path+'\'>Download%20as%20MP4</a>%20'+%20%20((navigator.userAgent.indexOf('Safari')!=-1)?'(control-click%20and%20select%20<i>Download%20linked%20file%20as</i>)':('(right-click%20and%20select%20<i>Save%20'+%20(navigator.appName=='Microsoft%20Internet%20Explorer'?'target':'link')%20+'%20as)</i>'))+'</span>';}void(0);
If you're having trouble copying that, go here:
http://www.themightyatom.nl/stuff/download_youtube_video.txtThen goto YouTube and watch a video.
Then click the newly created favorite.
A link called "Download as MP4" appears below the URL and Embed information. Use that to download the video.