If you paste "&fmt=18" at the end of any YouTube URL (up in the address bar of your browser), it will force YouTube to load the hires version of the video (if there is one). I think it’s using the H.264 iPod-compatible version of the video.

Example

Here is a lowres:

http://www.youtube.com/watch?v=4rb8aOzy9t4

And here is a hires:

http://www.youtube.com/watch?v=4rb8aOzy9t4&fmt=18

Makes YouTube almost watchable.