Will there be a post-aluminum material? Does the black-and-slate design anticipate something away from whitish, matte aluminum?
What's even lighter and stronger than aluminum? I'm no materials expert, so the only thing that leaps to mind is carbon fiber. But it seems to have developed its own aesthetic, and that's been sort of co-opted as a faux-luxury texture. However, if any company can be trusted to generate a new texture, a new aesthetic from a familiar material (that has desirable qualities), that company is Apple.
Update: oh look, seems like there's recent rumors in that area.