People using cases should be careful as some of the harder ones I believe can itself damage the paint. I know a person who has an iPhone 5 which apparently uses the same method of painting the aluminum black as the One. The case that he bought actually caused the paint to chip because it was metal and fits too tightly.
As for myself, my phone is still flawless. I will give one caveat though. The white bands on the back (I believe it is some sort of plastic) can get dirty and leave a marking because mine is a bit protruded.