Could you provide some detail on what method you are using to download the videos? MPEG Streamclip? Realplayer? It could easily be a glitch in your program.As of April 2nd, 2010 youTube has changed their code so that you can't download 720p & 1080p videos anymore. Has anyone found a workaround?
Did you try double clicking in the activity window while holding the option/alt key? Try that.Open HD video page in Safari.
Open Activity window.
Find URL of video (the one that's multi-megabytes long and probably still downloading as you're looking through the URL list.
Select the URL.
Option-double-click to download the file.
(Edit - I did the below steps initially before I realized there was a much shorter way to do it!)
Open Terminal window.
Type the following:
curl "<cmd-v>" > video.mp4
Wait as the video file is downloaded into your home directory and named "video.mp4" (or whatever you want to call it).