First of all, hand on heart, I do not know how that has happened.
1 month old and I honestly do not know how that has happened to it. Even this morning, I'm sure it wasn't like that. But saying this, I only noticed it when I turned it horizontally to play a game and that's when my finger felt it.
Now before fanboys leap to the defence of Apple and start accusing me of being a bad phone owner etc I've spent thousands of pounds on Apple products and that includes every iPhone bar the 4S since the 3G. I've also had iPad, iPad 2, 3rd gen iPad. Bought my GF an iPad Mini, between us we've had 2 MacBook Pro's & 2 iMacs. And I've never damaged any of them before.
Maybe I walked into something?!