The MacBook is only two years old but it is little more than a renamed iBook from when Apple made the decision to have "Mac" as a part of the name of all their computer products. For much longer than 2 years you've had the iBook/MacBook as plastic while its portable compatriot, the PowerBook/MacBook Pro, has been aluminum. It's just my opinion, but I still very seriously doubt that as long as there is both MacBook and MacBook Pro that the MacBook won't be aluminum.
I dunno, the MacBook and the iBook seem to be rather different creatures, despite being targeted at the same demographic. Whereas the iBook and the PowerBook had a rather considerable gap, the MacBook is nearly able to rival the MacBook Pro so long as you don't need a discrete graphics chip. Just remember, the PowerBook wasn't always aluminum.