Couldn't this be about a manufacturing process that will allow the casing to be more rigid while being more lightweight.
Which would be solid aluminum cases cut from single blocks, wouldn't it?
Unless you mean to imply that they are using some kind of injection molded plastic. I think the photo is clearly aluminum and the shine is the plastic liner that can be clearly seen covering the key holes.
Aluminum is much easier to recycle as well, so I'd imagine going the all plastic route would be much less "earth friendly" which is something they increasingly promote, both in design and packaging.
We all know Jobs has been focused on making products thinner, but you can only go so thin. Maybe the next frontier is in the weight.
Well as the products become thinner, they usually become lighter anyway. Less material usually = less weight. Unless they suddenly start using large quantities of lead in their case design, or increase the thickness of the aluminum (which the new manufacturing process seems to do the opposite), I think it's highly unlikely you'll see dramatic weight differences.
Form factor is the most important thing to them at the moment. The weight is already very good for their laptops, so any losses in weight will be the side effect of slimming the form factor, IMO. You also have to consider that if they do stretch out to a 16:9 form factore, that you have more surface area for components, which might alleviate the need to stack some on top of each other, allowing the bottom half of the clamshell to be slimmed.
The problem is that when you go lighter, you typically lose strength. Unless they suddenly decide to go carbon fiber, I can't imagine any injection molded plastic case being a better alternative to their standard aluminum.