A friend sent me a link to an YouTube video about some petty theft in Rio de Janeiro that had resurfaced for the Olympics. There are quite a few examples on that video what is going on in the city.
There have been some athletes that have had their belongings stolen (saw some US swimming team member yesterday being interviewed about their experience on CNN) and many are advised to travel only in groups.
I have certainly had my own experiences with petty theft in Brazil from several guys putting me on the ground and taking cash (in Salvador da Bahia) to police trying to shake some cash out from me when going from Ipanema to InterContinental (now Tulip) in a cab.
Then there was the case where Sofitel Rio de Janeiro hotel employee helped him/herself to some of my cash (read more here).
It is usually good idea to leave valuables in the hotel and have just cash and a credit card when going out.