How to keep footer always aligned absolute bottom

Take a look at the following page: http://www.savedumbo.org

My footer div does not attach to the absolute bottom of the page if
the window is stretched so the content div is more than the 700px I
have specified. How do I keep things as they are, but force the footer
div to always stick to the bottom of the page, even when the window is
stretched vertically?

Thanks in advance.