You can smoke in Rome with no problems. Marijuana laws are more laid back there than most of the US. A good source of information regarding marijuana laws and tolerance around the world is WeBeHigh. On the Rome page, it reads:
Quote Originally Posted by WeBeHigh.com
Legislation: Marijuana is illegal, but is tolerated for personal useage. A new law may be passed allowing up to 5 grams of any drug for personal usage

Law enforcement: Cops are very relaxed in Italy. They will aproach you if you are smoking in their face, but as long as you're in a crowded piazza, or in your home, they don't care at all. They are too busy flirting with girls and smoking cigarettes.
Have fun and enjoy your trip :stoned: