Your #footer style in the css file has
#footer {
position: absolute;
bottom:0;
height:22px;
background-color: #CC0000;
width:100%;
color: white;
}
bottom: 0; is often unreliable and may be affecting Opera.
position: absolute; bottom: 0; will not make the footer stick to the screen bottom while you scroll the rest of the page, for that you would need position: fixed; bottom: 0; which works in Firefox and IE7 but doesn't work in IE6.
Try deleting position: absolute; bottom: 0; as it doesn't seem to do anything. Without this code the footer should just follow the previous divs and scroll, just like it does now and may work better in Opera (I haven't got Opera so I can't check).
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.