Longhorn may not be here yet but you can start learning about XAML now.  XAML is a new mark-up language that Microsoft is developing that allows users to easily create user interfaces.

Stardock plans to incorporate XAML into DesktopX at some point to combine its power with DesktopX's native easy of development.  In the meantime, check out this article on it.

(update) External link fixed.