I've seen 2 arrested...
I worked for a big box retailer for a couple of years, from 2001 until about 2 months after they changed the uniform to that "geekier" thing (at that boint I gout out as fast as I could...)
Part of the training I had to give all my techs was that when they were working on a customer machine:
1) ensure no "explicit" images are displayed on the screen before connecting it to the overhead monitors. (I watched a guy get fired for putting a desktop on display to a crowd of people featuring a nude woman as the wallpaper)
2) when working on a PC, don't make an effort to search, but if you find evidence of child pornography or other federally illegal activity a) stop, b) notify management, and c) management will notify the FBI, and d) do not make the customer aware.
We had one PC come in, this guy was obviously an idiot, it was covered in stickers of naked 12 year old girls and one reading "if it's of age, it's too old!". We accepted it, logged his credit card and phone number, and called the police the second he was out the door. The FBI showed up, took it away (we never powered it on btw), and then called us back later. They asked us to tell him to come in to discuss the repair costs and pre-pay, and we set an appointment. The FBI was waiting when he came in and arrested him on the spot. Over 1800 images were found on his system and he got a sentance of over 25 years.
Another guy had his little girl (who he brought in with him to drop it off) pictured nude on his desktop. She was 13. We told him it would only take 15-20 minutes to add the RAM he wanted and to wander around the store for a while. He was arrested less than 10 minutes later. Some of the pictures they found on his machine had him in sexual positions with his daughter...
If you're going to have blantantly illegal crap on your computer, DON'T BRING IT TO A RETAIL STORE FOR REPAIR!!!
Per a member of the FBI we spoke to after the first arest, they told us that not only was it our duty to call the police, and they thanked us for doing so, but they reminded us that if we saw child porn on a machine that we ourselved could be subject to arrest if we DIDN'T alert the police. Our company execs confirmed that was the case. Not reporting an incedent of kiddie porn is itself a federal offense.
Now, i can agree, it;s possible that in places like your internet cache, tmp folders, and other places viruses stick images, you could very likely have illegal images on your machine you honestly know nothing about. When it;s your desktop image, and it;s you and your daughter, you have no case.... When you have a photo management app, and meticulously organized images, also sorted into clearly identifyable folders in the file system in common locations a user could not possibly overlook (like a folder ON THE DESKTOP called "13yrold golden showers"), you're guilty. As a tech, I've seen some pretty nasty stuff on people's computers (dump.gif), and much of it was actually in plain sight!
I've seen machines with web browser home pages set to illegal porn sites, e-amil archives with hundreds of porn photos shared with others, desktops with nasty images, chat apps that auto-launch and connect and then immediately get hit with invites to sexual chat, newgroup servers defaulted to alt.fettish feeds. People please... Provided it's legal stuff, the least you can do is make it so i don't have to see that crap on your PC when you bring it in for service... If you;re that depraived, don;t bring it in!