Shawn Blanc:

After using my iPhone 4 every single day for nearly a year and a half, the glass on the front and the back was still in near-mint condition.

Ditto here, in fact the most scuffed up parts were the metal banding and the annoying plastic around the very edge of the glass. My iPad 2 on the other hand has an aluminum back that is dinged and scratched and worn. Glass is clearly better for wear, whereas aluminum is better for dropping.