By what mechanism does male circumcision inhibit HIV infections?

I've read in a number of places, and just recently heard Bono say on The Daily Show, that male circumcision can inhibit HIV infection in men. This article in Scientific American says infections can be reduced by more than 57%. Why would that be? Circumcision and exchange of fluids seem more or less unrelated to my layman's perspective. How could a circumcision be effective at preventing transmission of HIV?

Thank you in advance, askscience!