Samsung-focused blog SamMobile reported last week that, "the sides are made from aluminium but the back is still plastic."
This report was seemingly confirmed by Russian mobile analyst Eldar Murtazin who also says it's going to be a plastic phone.
Some people were hoping Samsung would drop plastic in favor of better materials. Apple, for instance, has an aluminum and glass case for the iPhone 5. HTC's One is virtually all metal.
Nokia uses a polycarbonate body, which is a fancy version of plastic, but it feels much more solid than the plastic on Samsung's Galaxy S III.
Samsung has done just fine with cheap plastic, so maybe it doesn't need to change. But, for a flagship phone it seems like it should step up its materials.