YouTube tests a new border code-named Cosmic Panda. There are many cosmetic changes: videos are centered, player's wheel are now black, video thumbnails are a lot bigger, suggestions are displayed below the video, profile photos are displayed next to the comments, channels and playlists have a totally new layout.
Probably the most attractive thing about the new interface is a Chrome-only feature that lets you play a video in the background while you visit a channel. YouTube is more fluid and I wait for to see a similar feature when you perform a search and when you click "view all comments".
The new border can be enabled and disabled at youtube.com/cosmicpanda.