http://www.useit.com/alertbox/scrolling-attention.html

Design Implications

The implications are clear: the material that’s the most important for the users’ goals or your business goals should be above the fold. Users do look below the fold, but not nearly as much as they look above the fold.

People will look very far down a page if (a) the layout encourages scanning, and (b) the initially viewable information makes them believe that it will be worth their time to scroll.

Finally, while placing the most important stuff on top, don’t forget to put a nice morsel at the very bottom.