out of interest why do you want to download the videos? want to stick them into an adobe premiere project or summat?
i've seen people want to do this before, but if its just cuz you like the clip, add it to your youtube favorites its not going anywhere
(i was actually just thinking as i finished typing that maybe you'd want to download em for your iphone or whatever, but i've seem the iphone view youtube stuff straight off the site, so it wouldn't be for that reason)