Note: This was posted as a reply on another topic thread by Rome resident "Ron in Rome" (as he's known here). I thought it deserved it's own topic, since this question is asked so frequently.
By law, CDW and theft insurance is built into all the rental car rates in Italy. In addition, DO check with your VISA card as NONE of my cards (including a Platinum AMEX) will cover ANY rental car insurance in Italy... (no surprise with the number of car thefts and the way folks drive over here!)
I end up having to purchase Super-CDW. It's expensive, but driving here you will probably need the peace of mind. I rented a car a few months ago that got a door ding in a parking lot and they wanted to bill me over $1,000.... thank goodness I purchased the $0 deductible! DO NOT drive under-insured in Italy.