I had to fix this very problem. I have disassembled many dozens of cellphones, smartphones, PDA phones, and always prided myself on leaving them with nary a tool mark. However, I found it almost impossible to open my iPhone even after watching the PDAParts video on Youtube and reading that guide on ifixit. The PDAParts video makes it look ridiculously easy, there must be some subtle trick I'm missing. The plastic part comes out easily enough and is not a problem. The metal back, I had a very hard time removing. It was possible to pry up the back a little to get a case opening tool in, but that metal case quickly chewed up and destroyed my nylon case opening tool. I won't go into my horror story, but a sliced up thumb, a lot of blood loss, and 3-hours later, I finally had the thing open. It's got stress marks, tool marks, and didn't fit back together as well as before I opened it.
So what's the trick? Did I use the wrong tool? I used one identical to the one on
this site, not the green one sold specifically for iPods/iPhones. I also tried scrap credit cards with the edges filed down sharp when the iPhone destroyed my case opening tool. That works great for plastic phones, but the iPhone just chewed them up.