The only thing that has scratched my iphone was an otterbox defender case. I would take my phone out once a week to clean dust that got between the case and phone. After 3 months I got tired of this and purchased an otterbox commuter case. A thorough clean of my phone after taking it out of the defender case revealed two things... 1) the bottom part of the back of my iphone has 3 scratches- presumably where the phone slid into the bottom part of the defender case. 2) where dust would accumulate, there appears to be small divots in the glass that appear to have been created by the case pressing the dust hard against the glass.
I had my phone in the commuter case for another 1.5-2 months but got tired of cleaning the edges of the screen (under the case) every night. I've been without a case for ~2 months now and have not developed any additional scratches on any part of the phone. My phone is better protected "naked" than with a case that is supposed to protect it.
I take care of my phone, I put it in a pocket by itself (no keys), keep it away from dirt and grime, and don't drop it. I have not developed any new scratches from normal use without a case over a 2 month time - and don't expect to develop any with continued use.
the iphone is a well designed product - which will not scratch under normal use. Those who do have scratches on their phones and claim to not have done anything wrong clearly... at the very least... got sand in their pocket and put their phone inside (or some other story like that).
