After reading the article on Lifehacker about giving your iPhone a "brushed metal" makeover, I decided to try it myself. Now, I had previously done this same technique back when I had an iPod Photo, and it worked quite well (especially since I bought it off a friend, so it already had some wear to begin with). After getting some materials together, I was ready to start my brushing adventure!
After that, use a two-sided kitchen sponge (the green/rough side) until you see a smooth, brushed-metal finish. It may take some time to get the look you want, but when you're done, it's well worth it!