It's complicated...I've started on a template set that does this but was never happy with it nor did I finish it. There is a person on the list who has done it very well...but...he took away the possibility of having the 'index' scrollable. This is a smart move and makes life much easier.
Otherwise, you end up using alot of javascript to set the positions of the two divs. However, now that IE6 is being supported less and less maybe this is all more easy now.
That's what I know on the subject...
This is a good topic for the Indexhibit Laboratory that is taking place next week (costs 50 euros to sign up). I'll probably put the note back up later today...