I hope they are. I really don't like the current aluminum. That's no way the Air should weigh 3 lbs...In addition to the good posts by @Serban55 and @Krevnik, I would just add that Apple has released products with titanium and other materials but aluminum has, so far, them the best balance of cost, recycling, machineability, and heat dissipation (even for laptop systems with active cooling, if the chassis is thin enough the case material still makes a difference). However, pretty sure MR has published articles on recent Apple patents for the use of other materials including both titanium and carbon fiber so they are definitely exploring them with the caveat that no one know if or when they might switch from Al.
My preference would be carbon fiber, but we shall see when that happens. But I'd be willing to bet we wont, weight doesn't seem to be a concern for Apple.