According to Automotive News, a spy shooter in Colorado spotted a prototype for the next-gen F-350 testing. He happened to have a magnet on hand and got close enough to check the truck out. When he held it up to the metal in the bed, it didn't stick, which signaled to him a switch from steel to aluminum.
Obviously, this claim raises some questions. Given that it was a test vehicle, one possibility is that the Blue Oval is just evaluating the feasibility of switching to aluminum for the Super Duty trucks, not necessarily committed to it yet. Ford has been testing it quite exhaustively, after all. In fact, much of the rest of the truck in question was covered in camouflage, so it's possible that the magnet failed to work along the rest of the body not because it was aluminum, but because it wasn't powerful enough to get through the disguising material. Thus, the lightweight metal's use could be far less substantial than on the new F-150. Still, it was a clever idea for the cameraman to check things out and might have given us the first hint about brand's next heavy-duty models.