Oops. I initially wrote that your #logo needed its float:left; turned off, but I see now that I was incorrect. Your entire #logoWrapper is behaving as though it has float:left; enabled, though I don't see that explicitly set or inherited. I do see what appears to be a typo in your css where you've set the background image for your #logoWrapper. The IE developer tool is showing this under #logoWrapper:

background-image : url(../images/header_bg.jpg); TEXT-ALIGN: center

I'm not sure why the TEXT-ALIGN bit is in there. Are you missing a semi-colon or something?

-JimmyJazz