I have spent a lot of money on the best cases and not one has prevented the bezel on and one of the 3 iPhones I've owned (I've had one of each generation) from becoming scratched. As stated, dirt and grit from your pockets gets trapped between the case (no matter how form fitting) and causes the abrasions. This time around I took a new tact and taped all the edges of the bezel with scotch tape. I made sure that you could not see the tape as it is covered by the case, but it is there, hopefully protecting the bezel.
The only reason I care is that I've sold each of my previous phones to upgrade to the next and like to pass along a device to the next owner in as good as condition as I can.
Also, FWIW, I've found no case will ensure the back stays scratcj free either - for the say reasons. With my 3G it went into a case the moment I opened it and it did not come out for over 4 months. It was scratched to heck. or more specifically, it looked "rubbed." Tjis time I carefully cut the front off the protective film the phone ships with and left it on as I put the phone in the case.
-Calvin