I have a really hard time believing that if you took a key to it (like they did in the stress test) and apply a decent amount of pressure it would not scratch. The screen is not made of some ultra composite material that we have not heard of - it is simply made of glass. I guarantee you that if I try and key my window if you scratch.
Not willing to test this theory out with my iPhone, but I would think it would be simple physics when applying a sharp object with significant force to a glass surface.