Let me start by making clear that I'm not trying to argue for or against piracy. Nor am I trying to justify anything, make any moral argument, etc. This is merely an anecdote to add to the pile.
I have pirated software for years and continue to do so. When I was a kid, I pirated software because I couldn't afford it otherwise (and I mean that strictly; my family could barely pay the bills). When I went to university, I was on student loans, and had even less disposable income.
Now that I'm a professional, I buy a lot of software. I bought an XBOX 360, Rock Band 1 & 2, and a large portion of their music catalog -- right up until my XBOX got banned from XBOX Live. Now I don't buy anything for Rock Band. I've also bought several expensive programs, including a karaoke maker, special editions of some of my favorite games, and several $10 indie games that I absolutely loved. But I still pirate.
When it comes to choosing whether or not to pirate something, it mostly depends on what it costs. I can't afford to pay $175 for a copy of Windows, so I pirate it every time. I can afford $10 for a good indie game, and if it looks good, I'll probably just buy it without even trying to pirate it. For more expensive programs, I usually try a pirated copy first, and if I like it, I buy the real thing if I can afford it.
It probably seems strange that I would pay for something that I've already pirated, but as I said, I don't pirate just for the sake of pirating. I do want to support people who make software, especially if I think that software is worthwhile. Unfortunately, I can't afford everything, and even the basics (Windows, office apps, anti-virus, etc.) are a formidable investment. In fact, if you add up the cost of the software I use on a regular basis, it's probably worth more than my car (which isn't saying much for my car). The computer was expensive enough; I can't afford to furnish it with software.
But I do want to pay for things that are worth paying for, so I do pay for software at times. But if I can't afford them, I still use them, because (in most cases) they are still something I require. Sure, you don't /have/ to have a computer, but you also don't /have/ to have clothes that fit or food that's good for you. In my case, a functioning computer is an important part of my life (both in my leisure time and my professional time) and I would lose a lot if I didn't have it. But I can't afford the software for it, so I pirate it. If I had the money, I would pay (and sometimes I can and do). At least, if it's worth it.
I have also downloaded and used a lot of software that I would never pay a penny for. Unfortunately, before pirating was as easy as it is today, I often purchased this software, only to find out it was terrible. Now, I have the option of downloading it first, so I do. Especially for anything expensive. I probably wouldn't do this if I had the option of returning software I didn't like, but because that's not allowed (because it would make it easier to pirate it), I have to pirate it to try it out (ironically). And sometimes I end up buying the things, but more often than not, I don't like the software and I never use it again.
So, in my case, piracy doesn't mean lost sales. The things I like and can afford, I pay for. The things I can't afford, I would never pay for no matter what. The things I don't like, I would demand my money back, but since I can't even do that, I just avoid purchasing things all together for fear that they might not be worth the large investment. If somebody found the perfect way to prevent piracy, I would be exposed to, and therefore buy, a lot less software. I never would have tried and bought World of Goo, Machinarium, Power Karaoke CD+G Maker, or Jaikoz. For me, piracy lowers the risk of trying a new program; if I had to pay for it, the risk would be too high to try most things. Piracy actually encourages me to buy software.
So, here's one more anecdote to add to the pile. It probably isn't the typical case, but it's one to consider. If nothing else, it's evidence that these things do happen: people do buy software that they've pirated.