I had this same thing happen with my D7000. I was on vacation at Disney last pic camera took there was a black line diagonal across the frame. Tried everything you discribed, with no luck. I was a bit upset being I was on vacation. I did have a p&s for back up luckily. Sent camera in to Nikon and they covered the repair however you have to cover the cost to ship to them. I had never bumped or dropped camera and I had only about 6k actuations. Only 4months old. I think this is somewhat of a common occurance because there is a you tube video showing the problem. Took about two weeks to get it back. Seriously thought about trading it in for a 7D after getting it back. So far no problems. But would hate to have it happen once its out of warranty.