This is a pretty cool trick using only CSS to change a logo on page scroll. It utilizes the 'background-attachment' property in CSS.
The '#homelink' div is there for the logo link to make it clickable. The rest is in the CSS. When building it make sure the page scrolls or you never get to see the effect.
This is the minimum CSS needed to make this ork correctly.
#container { background: transparent url(demologo-wired.jpg) fixed no-repeat 30px 30px; }
#logo { display: block; width: 138px; height: 138px; background: transparent url(demologo.jpg) fixed no-repeat 30px 30px; }
#homelink { position: fixed; top: 30px; left: 30px; }
#homelink a { text-indent: -9999px; display: block; width: 138px; height: 138px; }