YouTube has been testing higher bitrate encodings of it videos, which you can see if you add a &fmt=8 or &fmt=16 to the video url. Historically, all videos have been delivered to the lowest common denominator: sorenson encoded 320×240. By adding &fmt=6 to the URL, the video is served up in 448×336 resolution and I’m guessing it’s using the VP6 codec (can anyone confirm?). &fmt=18 gives you the iPhone-style MP4 stream.
What videos will actually look better in the higher res format is completely dependent on the material that was uploaded to YouTube, obeying the rules of garbage in garbage out. I’ve looked at a number of videos where you can’t really tell the difference between the low and high-res versions, presumably because the uploaded video was already heavily compressed or pre-scaled to 320×240. There are a few, however, that are strikingly better, such as the skateboarding dog above.
A greasemonkey script is available which will cause Firefox to automatically load the fmt=18 version, if available. A quick install and you can be wasting time at twice the bitrate.