YouTube gave itself a nice little upgrade a short time ago, which no doubt some of you have spotted already. With the 4:3 aspect ratio mostly being a thing of the past, they decided they should get with the times and allow for widescreen (16:9) content to be uploaded and watched in full aspect.
As a result of this, the site's layout also gets expanded to 960 pixels so as not to cripple the video in any way. Furthermore, existing 4:3 content will not be stretched (rather, you'll just see two black bars on each side) so as to retain the original aspect.
A nice move, YouTube!