I agree that you should be careful, but having an iPhone and iPad is absolutely no problem.
Usually I have my headphones in, not just to listen to music, but because otherwise I don't hear my iphone ring with all the street noise. So I only take my phone out when I need to change the music or when i need to text. Yes, I text when I am out on the streets. It depends on which barrio I am in, and in which street. I would never take it out in microcentro, I am very careful on busy streets like Santa Fe. I wouldn't take it out at night. And I always hold it tightly with 2 hands s if it might fly away if I don't. If I do have to answer the phone I always take it at the side of the houses, to make sure motochorros can't take it. It seems exaggerated, but is is just a question of habit. Many people have iPhones (more have blackberries) but you don't see them on the streets. You see them when people call or text in restaurants, in tennis or other clubs, at private parties. They won't put them on the table in restaurants like they do in NY.
I have my iPad in my bag most of the time, I would never take it out on the streets or in the subway. (and as said, there is no space to take it out in the subway). But I do when I am in a bar or a restaurant. It is very common to see people with laptops and iPads in restaurants. Even when the door is not locked and they even sit in front of windows. They don't have old outdated laptops. Many have brand new fashionable MacBooks or notebooks. They all just put in in a bag or backpack when they leave the restaurant.
I had more people reacting on my iPad when I was in a restaurant in Europe then when I go out here.
We do have a very cheap phone with a prepaid card, which my son uses when he goes out. Just in case. Not only because we are afraid his phone would get stolen, but because he would probably defend himself and refuse to give it. And get hurt...
if you look like a tourist you are considered an easy victim. A friend of mine had her iPhone stolen in the subway, she was talking english with a friend, had a starbucks cup in one hand and her iPhone in her other. A very easy target.