It just happen i was looking today for a sim card in italy this summer the major player are tim.it; vodafone.it; wind.it and e new provider 3italia.it. The last one has the best offer as for now as price-wise. the other three have better reception almost everywhere. I was trying to complete the order with 3italia however not sure if they will ship in US, italian poste are known for not delivering and also not sure if you need to activate within a certain time frame since my travel plans are for mid july. i think tim has a store in the Milano malpensa airport. the best bet is to get it from a store in italy, italian law requires to register the sim and provide copy of passport.
sim nano = iPhone5 and above