I have also wondered about this. The new unibody construction should result in a lighter computer. Instead, the unibody 15" is a bit heavier than the "classic" construction. I think the glass is partly to blame.
Glass is very heavy material. This is evident with the "loose hinge problem" on the unibodies. There's nothing wrong with the hinge, it's just that it has trouble supporting the weight of a big ass piece of glass at a 45 degree angle.
The matte 17" version should be lighter because the glass is removed in favor of matte plastic, so it should logically be lighter than the 6.5lb glossy version. However, the sheet of glass is one of the reason the screen it's rigid and doesn't bend at all. So I think when Apple took off the glass of the 17" mbp, they had to find ways to reinforce the screen on the inside with additional materials, so it probably isn't noticeably lighter overall.