iFixit got an iPhone 4 early and tore it down. The company determined that the front of the iPhone 4 is made of Gorilla Glass although the back glass on the iPhone 4 is not Gorilla Glass and instead, some unidentified hardened glass.
Engadget managed to get hold of a review unit about a week ago and within that week with normal wear and tear, the back seems to have received a few scratches.
The scratches are difficult to see under normal light, but if you hold it against the light they are clear to see.
It appears that the iPhone 4 isn’t as robust as we hoped for although to be fair, every single phone I have ever owned gets the occasional mark here and there. By using hardened glass it appears that the scratches will be fewer than the older back on the 3GS.
That is why they make cases ! DUH ! Anthing as hard or harder than glass WILL scratch it.