We're expanding the width of the page to 960 pixels to better reflect the quality of the videos you create and the screens that you use to watch them. This new, wider player is in a widescreen aspect ratio which we hope will provide you with a cleaner, more powerful viewing experience. And don't worry, your 4:3 aspect ratio videos will play just fine in this new player.
As always, we welcome your feedback and encourage you to share your thoughts with us on this exciting, new change happening for all videos on YouTube.
Have fun watching!
The YouTube Team
[NFGB] Link - from YouTube Blog