Trailers still have to have the square back end available for matching with loading docks.
Here's an article on the 10.7 mpg Peterbuilt-Cummins supertruck project ... (part 2 is even more interesting).
Delving Into the Cummins/Peterbilt SuperTruck: Part 1 - Articles - Fuel Smarts - Articles - TruckingInfo.com