I've been to South America many times - Brazil, Chile, Argentina, Bolivia. Argentina was relatively safe until the recession several years ago - there are many "Express" kidnappings, where they will grab a person, bring them to an ATM and withdraw cash - both tourists and native Argentinos. So, I do bring an ATM card with me, with little cash in the account for emergencies and usually leave it at the hotel safe. Buenos Aires is a big city, stick to the tourist areas and don't change money on the streets - I've gotten a counterfeit note before. Change at the Cambio de Casa, never mind that the rate is slightly higher. Sometimes banks in Argentina gives better rates, but you need your passport to change. In fact, any country in South America - don't change from street money changers.
Don't walk around with a tourist map. If you have to ask for directions - go to a store or ask a police officer.
Santiago centro is notorious for snatch thefts. The first time I went 12 years ago, noticed that almost everyone, while walking, would hold their handbags, briefcases very tightly and close to their bodies. That's a good indicator that snatch theft is rife in the city. The news have shown camera footages of people getting their bags snatched at traffic lights or their backpacks slashed from the back. So I learnt to not carry expensive stuff in my backpack, carry only my driver's licence (for ID purposes), if I'm not planning to shop - no credit card, only little cash in a small wallet or coin purse and stuff into my front jeans pocket, sometimes even safety pin the pocket and a little bottle of pepper spray in my jacket pocket - wear good running shoes! White waiting at traffic lights, I walk around so no-one would snuck up on me to slash or snatch my backpack. A bit paranoid, but it makes me feel secure. When drugs, alcohol and guns are easily available in countries you visit, best to be a bit kiah su. I ask friends or the hotel (at each city) the areas to avoid - there are always some parts of the city that's more dangerous than others.
I've heard stories about suitcases / bags being stolen at the airport in Lima Peru, even in Santiago. They work very fast so always watch your bags and someone wrote about watching out for little kids - yes, watch for groups of little kids. They can surround you and strip you bare and there's absolutely nothing you can do. If you're at the internet cafe, always put your backpack or bags in front of you, at your feet or within your reach. ..you won't know it's gone until it's gone. The last few times returning from Chile via Argentina found the locks on my suitcases missing and the zipper tabs broken. Nothing was taken from my suitcases - but as super paranoid Mammas would say - they might put stuff in your suitcases so spend a bit of money at the airport and get your suitcase cling wrapped! A Chileno told me the worst airports are in Lima and Buenos Aires - stuff goes missing while in transit waiting for a connecting flight.
That's not to say you should not visit these countries - I've never had problems in S.America - even though I traveled many times alone, by bus or flying, speaking little Spanish or Portugese. Best to visit South America now before too many shopping malls or international coffee chains open up.
Always try not to look too much like a tourist. The only time I got mugged was in Italy and was able to claim from my travel insurance. S.America's a great place to visit, backpackers are a friendly lot - nice way to meet fellow travelers, however, unless you're going to a hostel, don't share a room with a stranger aka fellow traveler - have heard that people have taken off with personal belongings, cash etc while the other was asleep.
Was never into photography but bought a DSLR this year and will be bringing camera + accessories when I go to Chile this Sept, so now I am getting paranoid!!
My camera is on a Black Rapids strap and am thinking of wearing a jacket over it so hopefully my camera won't be so obvious and maybe carry a monopod with me...or hire a bodyguard? Ultimately it's better to loose a camera but keep your life...but I would surely like to whack the guy with the monopod first
Happy travels.