Just in case anyone hasn't heard, we've had two wikimania delegates have their bags stolen so far, so please be extra careful in BA.
WereSpielChequers
I would want to add that you should prevent specifically from pickpocketing. There are no special "physical" threats in Buenos Aires, but people used to catch tourists on the fly. So, two basic preventive measures:
* Never ever let your personal belongings alone, far away from your neck, arms or back (unless they are on the hotel, of course); * Try to prevent looking like a tourist. This sounds stupid but is basic. Avoid ultra-giant backpacks when walking on the street and don't use your notebook outside closed spaces.