Create a block that will scroll down the page as user scrolls

I would like to create a block that will scroll down the page as user scrolls down the page. My guess is that I can create a block in the admin>blocks that will follow the weights of the other blocks (this will be the "last" block.) and in the template.php setup some code that will check the location of that specific block and when the position gets to say 10px from the top of the window to set the position attributes to an absolute placement. At that point the block will always be say 10px from the top of the window.

Any ideas?

Stuart