Interesting question. If you think about the physics of air flow the front wheel is out there in the leading stream of smooth air. By the time the air goes past the back wheel it has been twisted and bent every which way by the frame, your body, the pedals, and everything else. So it would stand to reason that a slick wheel would get you more bang for the buck on the front than anywhere else.
I have an SRAM S60 wheel on the front and a standard CXP-22 on the back of one bike. I think it makes a big difference in the performance of the bike, but... I never tried it with the same wheel on the back.
From a "bling" stand point it is tough to beat the S60 with black spokes on a black bike. I will have to post a picture later.