[EcoVan]

Here's a side shot of the air dam/valance so you can see its shape. The upper portion mostly follows the curve of the bumper and the dam is actually the lower portion.

And yes, I know that aero design is very complicated... I've read Huchos' book along with studying NASA's 1970's Van research that lead to the Aerodyne and other aerodynamic semis, amongst other sources. In the last 10 to 20 years it seems like the whole auto industry is finally taking the subject seriously and instead of just making cars look aero, they are making them actually more efficient. Using air movement to improve aero ( such as the air curtains over wheel wells) is a big step forward.
However, if you take a look at these vans, it doesn't look like GM has done much with these vans since they were introduced. There is nothing to indicate any design work done on the bottom side, the wheel wells , the rear or just about any where else other than maybe the grill inlet and the addition of a small air dam under the bumper in the later years. The rounded edges front and rear are probably the biggest asset of the current design. There isn't even a rear "spoiler" to help air flow fill in the huge wake of the rear end. The body is essentially unchanged since its introduction in 1996, with the exception of some front end changes. Around 2003 the body was moved to a much heavier truck chassis.
To me it looks like it wouldn't take much to make big improvements to this vehicle... alot like the motorhomes. In fact this is a motorhome/ box truck chassis.