Sadly, I must inform you of two incidents that happened to friends of mine in the past week.
One friend was riding her new bike (which may have made her a target) in the morning along the beach in Leblon. She stopped to cross the street, and a fellow came up to her and tried to grab her bike away. She resisted a bit, and a couple who were walking nearby also came to her aid, they scared the guy off.
Another friend was riding her bike at 9:30 in the morning along the Lagoa, when a man sitting on a park bench jumped up suddenly, and stole her bike, knocking her to the ground. She was shaken up but not injured, thank goodness.
In both cases I am very glad that it was not worse, obviously, but it is still sad that this has happened. I will be much more careful on my bike... not that these women weren't being careful! I don't really think that there is anything they could have done to prevent these incidents, besides staying at home, and that's no fun.
So bikers, be aware!
As an aside, it seems as though many incidents in this area that I have been hearing of, have been happening around 9:30 in the morning, a time of day when I probably feel safest (and obviously shouldn't!).