Soon you will be able to watch people do strange things in high resolution video. While most YouTube videos are 320 x 240, all uploaded videos are actually stored in their native resolution in the YouTube archives. In order to efficiently play these existing and hopefully new high res videos, YouTube is testing out a new player.
This new player will detect the speed of the users internet connection and offer up a video of the appropriate quality. Hopefully the high res library grows quickly to make this worthwhile.