Almost any truss company can design and build a truss that will add more insulation depth and provide for a uniform R-value all the way to the outside edge of the walls. Such "energy trusses" add little cost to the overall price of a home.
Remember to ask for this very cost effective upgrade for a house with a lower slope truss roof.
(http://oikos.com/library/attic_insulation/trusses.jpg)
More information here: http://oikos.com/library/attic_insulation/index.html