Hi,
When I test with IE 9 using IE 8 mode it works fine, but when I try IE 7 mode it was not centered.
I was able to fix it by go to page settings and put a custom body class = gallerypage
Then in style.ie7.css I added this at the bottom:
.gallerypage .art-content-layout .art-layout-cell,
.gallerypage .art-content-layout .art-layout-cell .art-content-layout .art-layout-cell
{ float:none; }
I'm testing locally using an Artisteer 3 skin. When I try your site in IE 9 it shows as centered no matter whether I set compatibility to IE 8 or IE 7
Try clearing the browser cache in IE, go away from your site like visit google then clear the browser cache because it wonn't clear the cache for pages that are open in the browser.
Hope that helps,
Joe