Same exact problem, would love to hear a follow up when Apple responds.
Late 2012, Macbook Pro Retina 13". Daily use & carrying it to work daily. I think the screen pressures onto the keyboard and removes the coating. The thicker line is the exact area that lies between the trackpad and spacebar. It was always carried inside an extra crumpler notebook sleeve.
Image
(
full res image)
When the screen is on and you use a normal viewing-angle, you will barely notice it, only when there is sun and you watch from the sides it is annoying.
Had the PowerBook 12", Macbook Air 13" First Gen and now this one. Never had any problems. Until now
I am a Photographer, so I always used the micro-fibre cleaning cloth that I use for my lenses. I doubt that cleaning has anything to do with the problem. It is just the keyboard pressuring on the display.