It has a metal chassis. That is, the frame inside that gives it sturdiness. I believe they said this one was aluminum in the video (the one in the Droid RAZR is stainless steel). The outside is plastic on the GN, although the new material on the back of the phone should make it feel a bit less cheap, and the whole phone should be stiffer, which is good.
I'll be honest - while I'll happily take this over the glass back on the iPhone4(s), I wish some of the materials science that went into the RAZR made its way into the GN.
P.S. Am I the only one who hates, hates, hates the way the iPhone4 feels? It looks nice enough, but a flat glass back just feels wrong in my hand.