YouTube just added a new quality level, in addition to normal quality and high quality: 720p high definition! As such, I've modified another bookmarklet previously posted as a hint here so that it now downloads these 720p HD versions in H.264 QuickTime .mp4 glory, ready for use with iTunes, Apple TV, with no transcoding required.
To use it, just drag the bookmarklet from this page to your Bookmarks bar, then click it while viewing a YouTube 720p HD video.
[robg adds: For those interested, here's the source code of the bookmarklet:
javascript:if%20(document.getElementById('download-youtube-video')==null%20&&%20!!(document.location.href.match(/http:%5C/%5C/%5Ba-zA-Z%5C.%5D*youtube%5C.com%5C/watch/)))%20%7Bvar%20yt_mp4_path='http://www.youtube.com/get_video?fmt=22&video_id='+swfArgs%5B'video_id'%5D+'&t='+swfArgs%5B't'%5D;%20var%20div_embed=document.getElementById('watch-embed-div');div_embed.innerHTML=div_embed.innerHTML+'%3Cbr%20/%3E%20%3Cspan%20id=%5C'download-youtube-video%5C'%3E%3Ca%20href=%5C''+yt_mp4_path+'%5C'%3EDownload%20as%20720p%20HD%20MP4%3C/a%3E%20'+%20%20((navigator.userAgent.indexOf('Safari')!=-1)?'(control-click%20and%20select%20%3Ci%3EDownload%20linked%20file%20as%3C/i%3E)':('(right-click%20and%20select%20%3Ci%3ESave%20'+%20(navigator.appName=='Microsoft%20Internet%20Explorer'?'target':'link')%20+'%20as)%3C/i%3E'))+'%3C/span%3E';%7Dvoid(0);Mac OS X Hints
http://hints.macworld.com/article.php?story=20081125213451807