Update 2014: This article is very much obsolete and this script is no longer maintained.
Get YouTube Centered in your browser again. Requires Chrome or Firefox. And a understanding how to copy/paste this script into a new script editor in Tamper(Grease)Monkey.
So if you’re like me, and hate YouTube following the new google theme of “stick to the left” like I do, install TamperMonkey (I assume you use Chrome… everyone should…) and install this script. Firefox users, you can use GreaseMonkey.