Ok, this is the main div, it's all nice and such, and it should be static, not move at all. I am going to put a spacer in this one so that it grows a lot vertically.
Space
Now, this is the tricky part. This DIV should be static, but when I scroll the window DOWN, it should follow - but not fixed! No, not until the the browser window "hits" it should it start moving with the scrolling.

You can get an example of this if you surf to the Apple Store and pick a product, and then when you come to the custimize screen, you have a column (the price summary) to the right which follows the scrolling when the viewport "hits" it, but moves back into its place otherwise.