I have had a 640 naked for about a month without any scratches. My 1520 somehow got more scratches, maybe because the screen would get rubbed by my tungsten ring more often because of the size. Essentially gorilla glass is harder than most metals (steel, gold, copper, iron, platinum) but not all metals (tungsten carbide), diamond, or ceramic. That is why even a smooth diamond would scratch the glass, but a steel knife doesn't. It makes the videos on YouTube scratching a phone seem silly. Keys should not scratch glass anymore than chalk can scratch copper.
To the other reply about sapphire, it is harder than glass, but would still scratch from harder materials. The thing that makes it impractical currently is it is less flexible and prone to cracking. May change in the future.