I have seen it and it is much like an armored cell phone case; i.e. Otterbox or equivalent. Depending on one's hand and finger size, it might present a problem when trying to access some controls if you have large hands and considerable fingers.
As Farnog points out, the D300 is a stout camera and will nearly always work, but some people are compulsive over the appearance of their gear and for those obsessed with scratches and dents, the armor is worth it to them. An experienced photo-journalist usually has some of the most visibly abused camera gear in the world, while a hobbyist with a 15 year old camera, has nary a scuff on it.