The user enters the site.
At this point, the div is in the middle of the page.
As he scrolls down the page, the div first begins to move upward, but once it
http://jsfiddle.net/25rgq/
This is an old implementation of your desired functionality, which I've used. It's one of the first scripts I wrote, so JS & jQuery knowing people will most likely vomit. I wrote it mainly due all the examples I found online were centered on moving the DIV rather than setting it to fixed, and incrementally adding to the top-margin of the box resulted in really choppy movement.
Basicly it changes to fixed as the specified element is a custom set margin from the top of the browser window, and stops moving down once it hits a custom offset before our footer (we wanted to constrain the followbox to not move past our sidebar space).
Hope it is of any use, and if a plugin to handle this nowadays exists, you would probably be better of using it.