Why do you believe it will be aluminum?
Only because Apple has been on a "green" kick for a couple of years now and have touted aluminum as the most renewable resource there is for making computers.
Also, the addition of the plastic backing on the iPhone 3G was criticized by some for going against everything Apple has been making themselves out to be. Personally, I like the plastic backing of my iPhone. It looks slick and I've never had any of the cracking issues some have reported.