I said it before and I'll say it again...the laws of physics and chemistry that govern the universe we live in do not allow plastic to physically "scratch" steel. That being said, and speaking from both pictures I've seen and my own personal experience with my iPhone 4 and Apple's bumper, it IS possible, given enough time and friction, for the plastic to "buff" small spots on the stainless steel band, removing it's dull texture and leaving a dark, shiny spot behind that some may misinterpret as a "scratch". For those that say they can feel a scratch, by way of rubbing their nail over it and feeling it catch, I'll say again that it was NOT caused by the plastic bumper itself. Basic chemistry proves this. A piece of foreign debris with a hardness level equal to or higher than that of the steel MUST have been present between plastic and steel for an actual, physical "scratch" to occur. And it doesn't matter how well a person protects and cares for their phone. Unless it's kept in a sealed, sterile, particulate free environment, no amount of protection will keep debris off your phone.
G