Despite being glossy and shiny, the undeniable fact is that glass is not a good material to make products that are constantly being moved around, under stress, and in the hands of users. Glass breaks. That's why you never see products made of glass around you, except when it's completely necessary because the product itself needs to be transparent.
It doesn't matter that it is strengthened, like aluminosilicate glass, the one Apple uses in the iPhone 4's. In fact, strengthening glass to avoid scratching—which is what Apple did—makes it more prone to extreme shattering on shock. The reason: Aluminosilicate glass has a much higher internal tension than regular glass. What makes it harder also makes it more fragile.
Cases of broken iPhone 4's backs are already appearing. One of Gizmodo's interns broke his iPhone 4 after accidentally dropping it while testing it. This hasn't changed from previous generations. Hell, I broke my iPhone display twice. The fact is that, at the end of the day, dropping the phone while handling it is something that everyone will suffer sooner or later.
But the difference is that the iPhone 4 is all glass. If you drop any other phone, you have a 50% chance of breaking its screen. With the iPhone 4, the risk will always be there, no matter how it falls. It's just more exposed to damage because of the material choice.
Some people argue that the shattering doesn't matter. That the important thing is that this glass is hard to scratch. But, as GDGT editor Ryan Block showed, this doesn't mean it's scratch-proof. He scratched his iPhone 4 accidentally, without even noticing.