There is a sidebar 160px wide. a div with width:100% is then 160px wider than the browser window, requiring constant use of the horizontal scroll bar.
I tried a lot of things until the width:100%-160px which dreamweaver flags as unsupported but it works, check it out at www.AllThingsAquatic.com
You will see by looking around that all pages with enough text to wrap around display the background image correctly, but the others have it chopped somewhere around the middle of the page. They all use the ID=mainContent.
OK, I've come up with some kludge. It is not elegant but it kind of works. I created a div as the first element inside of the mainContent div. In there I put a few "The quick brown fox ... " with a color:white (that you cannot really see over the backgound). I used a top:0 attribute on this div. You can see the result at www.AllThingsAquatic.com sidebar menu "Products/Fountains" then click Outdoor on the to menubar.
There's got to be a better way to force the background image to fill up the browser window (viewport).