I have been on the official site, trying to buy tickets for the Colosseum and the Roman Forum: http://www.coopculture.it/en/colosseo-e-shop.cfm
When filling in the requested personal information, you are asked to specify your nationality, using a drop-down menu with a long list of countries. The strange thing is that the United States is not on the list!
I think this is not a big deal, in that I can probably call and arrange tickets. Just thought I would pass this odd result along. I would be interested to hear if other people have noticed the same problem.