I had a bad experience a few years back with a Toshiba that convinced me to (1) let my computers cool down a bit before closing the lid and (2) always put something like a thin microfiber cloth between the screen and keyboard. The time you spend packing stuff up, going to the bathroom, or finishing off your coffee instead of closing the computer and stuffing it in your bag can make a difference.
In the case of the Toshiba, apparently they designed it to blow excess heat out through the center of the keyboard. This melted the screen. I had it replaced once and the same thing happened again a few days later;-- a permanent horizontal line stretching across the screen.
Apple designs their computers better, and obviously the MBAs don't get hot like the MBPS, but people have recorded temperatures over 100 degrees on the keyboard. I hope this line on your computer isn't the same problem I had with my Toshiba, but at the very least it seems a good idea to put something between the screen and keyboard before in order to keep off oil and dust. My cloth only cost a few dollars and doubles as a screen cleaner
