To download HD from youtube, go to HD video start it & stop it (just to make sure you are in the right place) and then paste this code in the address bar, hit enter, dialog box will pop up to ask you if you want to download file: YES. then RENAME FILE in the Windows folder!! Because it uses the same name and will be overwritten.

Here it is for HD format:
javascript:window.location.href = 'http://youtube.com/get_video?video_id=' + swfArgs['video_id'] + "&fmt=22" + "&l=" + swfArgs['l'] + "&sk=" + swfArgs['sk'] + '&t=' + swfArgs['t'];

Here it is for HQ format:
javascript:window.location.href = 'http://youtube.com/get_video?video_id=' + swfArgs['video_id'] + "&fmt=18" + "&l=" + swfArgs['l'] + "&sk=" + swfArgs['sk'] + '&t=' + swfArgs['t'];

Will add more information tomorrow.