Another reason to center a site's content - it seems that some people could not stand having a huge chunk of white space, and must resize their browser before to eliminate the white space before focusing on the content.
From Choosing the Right Search Results Page Layout: Make the Most of Your Width:
In my field research, I’ve observed people’s reactions to the large, empty spaces that appear on the right in fixed-width layouts. All that space devoid of content causes what I can only describe as pixel agoraphobia. When that space first opens up, people usually grimace and attempt to reduce the width of the window to remove some of the empty space. Most Windows users who have maximized their browser window do this by clicking the Restore Down button on the window title bar. This sometimes results in a window size that is too small for a site, so users must then adjust the window size manually. When your customers are busy fiddling with the width of the browser window, they’re not shopping or looking at ads on your site. In fact, they’re usually becoming more and more irritated—and that’s before they’ve even begun interacting with your Web site’s fabulous functionality!