https://marco.org/2009/08/17/heres-a-nice-design-tweak-or-side-effect-of-the
Here’s a nice design tweak (or side-effect of the unibody panel): the screen is inset slightly from the rest of the bezel. The combination of the black rubber gasket, this inset, and the increased rigidity of the unibody shell should significantly reduce or eliminate the keyboard-imprint-on-screen problem that was so prevalent on many previous models.