Looking at this
image, that looks like plastic to me. Also, the thing weighs in at over 8 pounds, so I doubt they would cover the exterior in metal, which would just increase the weight. I could go rambling on about how if it was covered in metal, it might get hot enough to act like a stove top on your lap or the dents that could easily show up from the smallest of bumps. So I'm going to take an educated guess on this and say it's plastic.