I had the same problem with my 2011 17" MbP. I simply cut a 2.5 inch strip of paper the width of my screen, folded it about .5 inches along the edge and placed it along the top back edge (below the screen, above/behind the keyboard) essential making a little manifold that diverts hot air from the fans away from the screen. Make shift, I know, but it seems to have solved the "dark spot" issue when my machine is running under heavy load.