We were in Barcelona last month and I found the pickpocket paranoia that I'd read about a bit overdone. I'm sure there are pickpockets and if you're really not smart about stuff you could get ripped off, but we didn't encounter any conditions that I'd even consider favorable to a pick pocket, let along have anything happen. First, don't look any more like a tourist than you absolutely have to but, given that you will to some degree, don't look like a RICH tourist. We just wore tee shirts and shorts and sandals and nobody hassled us. There were enough tourists walking around in fancy travel clothes with tons of jewelry and fancy watches and extremely clean sneakers and such that just stick out so obviously. And buy local baseball caps when you get to town, rub 'em in the mud for a couple of minutes, and you won't get a second look. And there are shoppers running around loaded down with big name brand shopping bags. Just don't be one of those and you won't be the easy target.
In terms of camera bags, use one that doesn't scream "EXPENSIVE CAMERA GEAR" and I strongly recommend wearing it in a sling fashion, with the strap over your head and resting on the opposite shoulder from the side the bag hangs on. This way it will be very difficult for a thief to just grab it and run unless they actually want to bother with cutting the strap to take it. If its just over your right shoulder and hanging on your right side, its pretty easy to make off with - they'd rather not have to try to get it over your head. There's a lot of low-hanging fruit walking around - don't be the lowest hanging fruit.
And plain looking bags help. I had a Domke F6 that I carried all over Europe with an Ep2, and EPL1, a lens on each, two additional lenses, and the usual accessories (batteries, memory cards, filters, etc). I had the ruggedwear version which just looks like a fairly dumpy brown bag after a little use. Never had a bit of a problem, even in places like Florence and Rome which seemed MUCH more pickpocket likely than Barcelona ever did, just based on the overwhelming throngs of tourists in those cities. Barcelona was a piece of cake in comparison.
-Ray