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 to eliminate the white space before focusing on the content.

From [Choosing the Right Search Results Page Layout: Make the Most of Your Width][1]:

> 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!


  [1]: http://www.uxmatters.com/mt/archives/2009/03/choosing-the-right-search-results-page-layout-make-the-most-of-your-width.php