First of all, please don't do something like that, I don't know anyone that likes those things.
Something can be kept at the top like that using the CSS:
position: fixed;
As for specifying the URL, I'm not sure exactly what you're asking. When you click on a link on digg now, the destination URL is something like this:
http://digg.com/d1oIyx
The "d1oIyx" would just be used as a primary key into a database where they're storing the URL to be loaded in the bottom iframe.