The iPhone 4 has 326 ppi and I just took off my glasses and > I really had to focus until my eyes hurt to be able to see a single pixel. (I failed, I couldn't see one) Also I have to get really close on to my PC screen to see single pixels (235 ppi) > Now my question is, how could my retina have 530 ppi if I can not identify single pixels on a 326 ppi device?

I guarantee you can. Make a nice white image and place a few single pixel black dots on it. You'll see them just fine.

Or, for that matter, dead pixels.