Umm... it's aluminum. You know, the dull looking metal that's used as the main body material on...
Mac Pro
iMac
Mac Mini
MacBook Pro
MacBook Air
XServe
Apple Cinema Display 30"
Apple LED Cinema Display 24"
iPad
iPod Nano
iPod Classic
iPod Shuffle
iPhone 2.5G
Apple TV
Apple Remote
Magic Mouse
Apple Keyboard
Apple Wireless Keyboard
Or, to put it simpler, every current hardware product from Apple except Time Capsule, AirPort Extreme, MacBook and iPhone 3GS.
The metal you're referring to is the chrome bezel on previous iPhone models. Chrome has been phased out of Apple's product line on everything except the soon-to-be-retired iPhone 3G/3GS; they used to have it on the back of iPods up until 2005 which is probably why they put it on iPhones.
The only product left still sporting the polished metal look after the new iPhone has been released will be the iPod Touch, but I imagine they'll squash that anomaly like a roach by releasing a new model in time for the next holiday seasons.