Snow Peak is a Japanese company that specializes in minimalist titanium tools for outdoor expeditions. Recently, they released a titanium iPhone 4 cover. Made from a single piece of titanium, the case weighs just 0.6 oz, and provides your iPhone 4 with a harder-than-steel protective back surface. It sells for about $120.
You can also find iPhone cases made with CNC machined aluminum — Vapor Pro cases.