Ahh...I thought that was your site but I didn't want to assume. I've seen it several times and I think you're doing a great job with it.
OK...Now to your problem. It may be due to (in body CSS):
Code:
margin: 5px 10px 10px 10px;
For fluid layouts I'd use a top and bottom margin while setting the left and right to auto if anything. Defined margins are more useful for centering or fixed layouts.