Well as most know, the youtube app when on wifi gets high-quality videos from youtube's servers in mp4 format. I have found these videos to be even higher quality than the flv files on YouTube.com that I normally download and convert. After poking around I was able to get the url to the mp4's that I had bookmarked but this was still a hassle so I made MxTube (
www.MxWeas.com/MxTube). Basically, its a youtube search engine that allows you to download the High Res videos onto your computer pre-encoded. I have a regular and iphone version of the app, the regular lets you download the mp4 (wifi) or 3gp (edge) videos and the iphone version lets you choose your own video quality and can play the videos in portrait.
Max
PS: For more info on it look
here