I don't think that there are any big cities out there that don't suffer with petty thieves and they don't always target tourists - even locals get caught occasionally.

I was sitting on the terrace outside a pub near my office in London last summer and one of my friends got caught. There were four of us (two blokes, two ladies) sitting around a table and one of the girls had put her handbag on the table between us all after going to the bar to buy a round of drinks. Within 10 minutes of sitting down she noticed it had gone - literally from under the noses of the four of us. The fact that someone pinched it from middle of four people facing each other just shows how clever they are. None of us can recall any diversion tactics and no-one came to talk to us to ask for a lighter (a common tactic with pickpockets).

Laptops are also commonly lifted in bars in London - one of my colleagues caught someone trying to walk off with his laptop bag from a bar a few weeks ago. The thief just handed it back and claimed to have picked up the wrong bag but then walked out the bar empty handed so clearly was lying. The surprising thing is that he was wearing a smart suit and blended into the crowd seamlessly. Even the police would have had a hard time picking him out as a thief.

But from personal experience Barcelona is the worst for petty crime - I know several people who have had the unfortunate experience of having their wallets stolen there.