Ever since the first gen iPhone, it seems the case has become more fragile; plastic followed aluminum, glass after plastic. Those reminiscent of the days when the iPhone was cased in element 13 can buy this sticker, which covers the glass back of iPhone 4 with faux aluminum. This sticker reportedly offers scratch protection and is completely water/grease-proof. It even has a cutout for your mirror-finished Apple logo.
Available at iStickr for $9.99.