People have noticed that some circumcising countries have lower AIDS rates than non circumcising countries in Africa. However comparing USA (a circumcising country) to Europe, Japan, Australia (non circumcising regions) you get an opposite result.
Finally someone has made some sense of all this here.
It's a classic causation vs correlation confusion. Circumcision advocates were touting it as a great way to prevent the spread of HIV, seeing the correlation. However, in this study, they find that the biggest driver of HIV spread is really the infection rates of prostitutes, and the percentage of the population prostituting and patronizing prostitutes. The causation of the circumcision correlation is revealed to be that circumcising countries are generally more religious, so less women prostitute and less men patronize them, per capita.
So treating, educating, protecting prostitutes will net a far greater gain in HIV control than mass circumcision.