A little more info ...
I was in Italy in October 2012.
About a year ago, the rental car company sent me a notice (entirely in Italian) and charged me for an administrative fee for giving my information to the police, I disputed that charge with my credit card company and after they investigated they decided not to issue payment.
I just got the notice from the Roman police (in English) via certified mail this week. There is no mention of the consequences of failing to pay the €105 fine.
Lots of varying advice online, pretty much all of it relates to driving in Italy.