Lets not forget the Cd is only half the equation.
The Volt is supposedly 70.8 by 56.3 inches while the Prius is 68.7 by 58.7 inches. But, the Prius does not have the airdam we've seen on the most recent Volt pictures. So overall frontal area might be on the Prius side.
I am assuming the way they measured the Cd is different than the way it's normally done. Maybe they tested in a rolling wind tunnel, that can make a significant difference. But they never said anything so it's not worth anything anyway. The only thing I'll keep from that is that GM says the Volt has a 0.28 Cd.
The new airdam probably helps the Volt do better with 17 inches wheel than the Prius, since it's keeping a lot of air from going underneath and interacting with the draggy wheels.
Oh and btw am I the only one thinking the "aero neutral mirrors" claim is BS?