I think it would just mean that the aluminum would no longer be one of the differentiating factors (it's not like people confuse the iMac and MacBook Pro because they both have aluminum cases now).
Apple may of course revise the MBP down the road with some new case material (carbon fiber maybe, like Sony). Or they might do something subtler; maybe black anodized aluminum (which I'd love to see, frankly....... a return to the black PowerBook days).