I agree with much of what other posters have said, e.g. don't keep anything in back pockets, stay alert, etc. Rome is a great place, the people are great, and I've been back many times.

That said, Rome is the only place that I have ever been pickpocketed. I'm a fairly aware person, and have been to dozens of countries around the world, both first and third (and fourth) world, and have lived in Europe. My wife had her credit card and driver's license in her purse (aside from lipstick, that's all she was carrying), in the metro. The metro wasn't crowded; we made the mistake of relaxing our guard for a bit since the only other people near us were well-dressed, normal seeming people. A couple, in their late 20s, were near us, and as we left the train, one of them got my wife's cards. My wife knew immediately. There was no "distraction", doll/mustard/newspaper etc. They were pros. So don't be fooled just because someone doesn't "look" the part.

Got back to the hotel, and they knew the routine, called the credit card company for us (charges were already on the card, less than 45 minutes later), etc. The hotel staff said this happened very frequently.

From talking to other frequent travelers, most consider Rome to be pickpocket central for western Europe. YMMV. Only carry what you can lose/replace. But don't make avoiding pickpockets a preoccupation; otherwise, you won't enjoy one of the world's greatest, most enjoyable cities.