Apple originally used a plastic back for the iPhone, then they switched to a glass backing for the iPhone 4. The glass back coupled with the antenna issues in the iPhone 4 did not bear well for the device, and it appears that Apple is considering using an aluminum back for the iPhone 5.
A report from the Taiwanese Economic Daily News claims that Apple is abandoning the glass backing from the iPhone 5 in favor of a stronger aluminum covering. The source also suggests that Apple's new antenna will be inside the device's rear Apple logo...