It seems to me that someone who knows how to create docklets for ObjectDock could create a docklet that resembles Leopard's stacks.  I personally don't have a clue about how to develop docklets, but looking at the weather docklet with its 5 day flyout, it would seem to me that it would be not to dificult to modify it into a variable application launcher flyout that was customizable.  I may be wrong about that though.  I have never tried to create a docklet or even played with the SDK.  Please correct me if my thoughts are way off base.