http://youtubedownload.altervista.org/
Download this proggy. Its called Youtube downloader. It does just that. Right click on the url of the video you want to download. Then copy and paste it into the down load bar of the app. It'll ask you where to save it and you tell it .
It does some simple file conversions. So far anything that i've downloaded using this app, from Youtube is playable on the i pod and iphone.