Full disclosure: I haven't been paid for my photographs for years and years - since the days when I used to write magazine articles and they paid me extra for supplying my own illustrations. These days it's purely for my own amusement.
To the best of my knowledge photography in a genuinely public place is permissible: just bear in mind that members of the public don't often know this and neither will the only-Spanish speaking police officer they call over to take their side. I never -ever- photograph military locations without asking permission first. Having said that, they are invariably delighted that anybody is interested enough to take pictures there.
I've never -ever- lost a camera (famous last words?) but whilst I take very great care of all of them I always consider them sacrificial - remember the tourist in Plaza San Martin a few years ago? Just hand it over if someone tries to take it. I regularly swap film backs or memory cards so that if the worst happens I've only lost maybe the last hour or so of work.
If it's not something you've practiced recently, polish up your ability to "palm" a film or a memory card. You never know when that might come in useful. I was taking some pictures in Provincia of a building which had collapsed during construction when some people became very unpleasant about it. I suspect they already knew about the substandard materials which had been used. In front of them, and very visibly and very reluctantly I took the memory card out of the camera, handed it over and walked away - fast. What they hadn't seen was that, at the first sign of a problem with people taking too much interest in me, I'd already whipped the memory card with the pictures out of the camera and into my pocket and slapped in a blank replacement. Always be aware of your surroundings - but then I don't need to tell a photographer that.
It's not all collapsing buildings: mostly these days it's great architecture, old trains and people enjoying themselves.
