There is a bug in the global SE CSS. If your browser window gets smaller than 982px, the container div that holds #header continues to shrink even though #header maintains a width of 962px. The result is a background that doesn't quite reach all the way across the content.
The good news is that this bug is present on all sites (officials, metas and betas) and is cross-browser compliant and renders the same result in Chrome 17, IE 9, Safari 5, FF 10, and Opera 11 (all on Win 7) so you get high marks for consistency.
Potential fix: add "min-width:982px;" to .container in all.css