In announcing a deal that seems to bring aluminum ever closer to a major role in automotive design, Detroit's Ford Motor Company and Canada's Alcan Aluminum Limited quite fairly described the increased use of the lighter-than-steel material as being good for the environment.
The principle at work here is quite simple -- aluminum is a lot lighter than the steel it usually replaces, and less weight requires less gasoline and less gasoline creates less pollution.
What Alcan and Ford did not mention but which should not be forgotten by consumers who like a little juice with their daily drive is that more aluminum can also mean more performance. This I learned a couple of years ago, when I drove around Detroit in a Ford Taurus made almost completely out of aluminum and powered by the high-powered V8 that ran the SHO Taurus in those days.
That engine in that vehicle was already pleasingly quick, but with the aluminum body it was a major rocket, as people in hotrod Mustangs and Camaros and everything else learned to their chagrin on Woodward Avenue. That's what happens when you take about 700 kg out of the curbweight of a 6-passenger family sedan.
Behaving in such a manner does indeed have a counter-productive affect on the environmental benefits of aluminum, but quick launches is a relatively minor part of any car's normal life cycle so there would still be significant pollution benefits overall.