These are becoming more common on Autostrade, and are difficult to see in advance. Often hidden behind a traffic sign, they record speeding vehicles and fine appropriately.
Pavement mounted speed cameras
Pavement (sidewalk) mounted speed cameras are becoming more common in urban areas of Italy. Coloured bright orange or blue, they monitor the passing speed of traffic and record licence plates where the maximum permitted speed is breached. Typically they patrol 50 kilometer-per-hour zones (See picture and click for enlargement); there is usually a warning that cameras are in operation. The image shows a sign stating "Controllo Elettronico della Velocita", meaning Electronic Speed Control.
Opinion differs as to the tolerance of these cameras (assuming that your speedometer is accurate). Estimates range from 5kph to a maximum 10kph over the stated limit; however, I suggest you err on the side of caution and avoid going more than 5kph.
When you hire a car in Italy, normal practice is for the company to swipe your credit card to ensure against additional, unintentional costs. Should you happen to get caught by a speed camera, then the hire company will deduct the fine from your card.