It's from the screen pressing against the keyboard/trackpad when it's closed. Do you put it in a backpack with books in it?
I had the same problem and they replaced mine. It was only 5 weeks old when I brought it in. How long have you had it?
You should make a genius bar appointment ASAP.
I only put into bag once a week and no books in my coteetciel messenger. I don't think this is caused by pressing because I really handled it properly. Do you get this replaced on the spot at genius bar? Or they ask you to come another day to replace it?
Any luck with Apple, OP?
Had the same thing happen to my 2011 13" MBA. Just went back to best buy and showed it to them and they returned it and gave me a new one in box from the shelves, with the same 250 dollars off black friday deal that i got for it and it was 3 months old, but they had that 14 days after christmas return thing, so i had them switch it out. I ordered a radtech screensavrz (recommended to me by other macrumors posters) and used the stock white thin sheet until the screenzavrz shipped. Got it in blue for 15 dollars (including shipping because of a coupon), and all i do now is put the screensavrz in between the keyboard and the screen before i close it. Very easy to do, and i carry it around just like i did before i got the screensavrz, and after 3 months, no scratches. I put it in a thin incase sleeve to protect it from scratches, then stick it in my book bag with multiple, thick, textbooks. It's a great buy.
hear are the coupon code(s)
Well worth it IMO. Got the blue one, and it looks striking, and I use it as a cleaning cloth when my screen gets dirty because it is made from microfiber.
Attached a picture.