The iPhone 4s was the last iPhone to have a glass back. We've only seen iPhones designed with strong aluminum bodies since then. But with the next iPhone, we may see an all-glass design. Of course, if Apple chooses to go this route, it will hopefully have figured out a way to improve the strength of the glass used on its iPhones, especially considering that it moved away from a glass design in the past due to the iPhone's vulnerability to nasty cracks. This is another new feature that is expected in the Plus or premium model of the next iPhone.
Concept art by Imran Taylor for ConceptsiPhone.