My iPad 3 (launch unit) has a similar mark on the side rather than a corner, and a very small chip in the aluminum on the other. The screen didn't have any backlight uniformity problems or dead/stuck pixels though so I decided to keep it as I had to swap several iPad 2s to get a good screen.
I don't have any plans to sell the device—I will likely keep it until the iPad 5 and then pass it on to a family member as I did with my old iPad 2.
If I did plan on selling it to pay for an upgrade next year, I would have had it swapped for a perfect one in-store, and immediately put it in a full body case or skin to prevent any damage from occurring.
As it is, I use my iPad without any kind of case/skin because I don't like the additional bulk or the way that they feel on the device, and if it gets marked up, well that just adds to the character of the device. As long as there isn't any damage to the screen, I don't care how the back looks. That's not to say that I am careless with it though.
For what it's worth, Apple products use a bead-blasted finish, and are not anodized.