It depends on the application. During the winter when you're driving on snow or plowing then extra weight in the truck bed over the rear wheels helps traction because a pick up is naturally lighter in the ass end due to the bed of the truck being empty/lighter than the front. For your everyday truck owner that drives his truck to and from work every day, and to and from the store and only hauls things on the weekend, then the lighter weight will be a bonus because it will not only get better gas mileage but also a lighter curb weight vehicle is actually easier on the drivetrain parts. So your brakes, bearings, axles, shocks, springs, ring & pinion, motor, trans, transfer case, all that stuff will last longer because it is not having to carry around as much weight.
Heavy duty trucks drive poorly when they are not loaded down because they are equipped from the factory with higher spring rate springs and shocks that are set up to cushion more weight. So basically they are set up to support a full load since they are designed to carry heavy loads since that's what the HD trucks will usually be used for most. I haven't seen which truck models will have the aluminum bodies but I would guess most likely it'll be the 1/2 ton models and I would think the HD 3/4ton and 1ton models would probably keep the standard steel body.
But the fuel economy increase would offset any added pollution in the build process. A lighter truck is going to use less fuel over the course of it's life on the road and less fuel demand means less pollution.