Wordpress Plugin - Box that Stops While Page Scrolls

I would like to use your expertise to create a box that will scroll with the page, but stop when it reaches the last element on the sidebar. An example of this in action is over at [login to view URL] - As you scroll you'll see it stops with the Techcrunch box and you can continue to scroll on the left side without changing anything on the right. I'm not exactly sure what this type of style is called, but I'd love to have it in plugin format where people could upload their own images or place widgets like text and such into it.