Rental car agency will likely charge your credit card.
If you were driving a car with a foreign license plate, chances are the next time the owner of the car is caught speeding (or the next time it goes through customs, if non-EU citizen, just like when you get fined in the US and try to return without having paid the ticket) he may pass a hard time and have a talk ot two with Equitalia agents or Fiscal police, not to mention the increased amount because of the expiration date of the fine.

At the moment, there's not reciprocal forced collection inside EU and EU citizens tend not to always pay such "foreign" tickets, but if I'm not wrong things should change in a few months.