As someone who has worked in the tourist industry, I've had to deal with my fair share of tourists who have been personally robbed.
It happens everywhere. I even had one young woman have her passport stolen at the border crossing in San Diego (U.S. side.)
I've seen pickpockets in the NYC Subway, the London Tube and the Paris Metro. I've seen pickpockets in Rome, Paris, London, NY, San Francisco, Los Angeles and the Grand Canyon just to name a few.
As stated, they congregate where tourists hang out or where people might be off their guard.
Some people are lucky and never get themselves pickpocketed...ever. But that doesn't mean it will never happen to you.
When I travel, I wear a moneybelt. I also take extra security with my possessions since I'm usually walking around with some type of electronic device--cell phone, ipod, camera, etc. (Once you've had to deal with the process of trying to replace a passport and other forms of id's and credit cards, you think twice the next time.)
I'm also aware of my surroundings. Am I paranoid about it...no. Just aware.
There are many types of moneybelts with the most common being the types you wear around your waist, your neck, your leg, or connect to a belt and hangs down inside your pant leg or skirt. (My personal favorite as it's hardly felt. )