What's really important is not just that the back is made of glass, but also that the edges of the glass on both the front and the back are exposed. So if it falls, it will always land on one of its glass surfaces, and no matter how Gorilla it is, concrete will make the edge crack. With the previous iPhones, the edge was enveloped by plastic or metal, protecting it.
I think this problem can be solved without a major redesign, all Apple has to do is make the steel edges of the phone protrude over the glass and maybe a bit beyond it, a bit like the current MacBook Pro's screen. That way, if the phone falls anywhere except flat on the front or back, it will be protected by the steel.
However, the Bumpers do just this, so I'd say people should use those instead of other cases. I think Bumpers should be bundled with the phone when you buy it, it's a very important thing especially with such a fragile device.