With an admittedly small sample size, I’m seeing a lot more broken or spider-webbed iPhone 4 screens than I ever saw with the original iPhone or iPhone 3G/3GS. I believe this is due to the change in the form factor that raises the glass on the front and back of the iPhone 4 so that it protrudes beyond the phone’s metal rim/antenna.
This is a change over the iPhone 3G/3GS which had a plastic/metal edge that wrapped the glass screen.
The original iPhone had an even larger corner area between the edge of the device and the glass.
I believe that the design differences are likely a large contributing factor to increased screen breakage in the iPhone 4. When dropped, an original iPhone or iPhone 3G/3GS that lands on its corner would land on metal or plastic. When an iPhone 4 is dropped and lands on its corner, there is a good chance that the glass is impacted before the metal rim/antenna.
I’d love to see actual stats on this, I’m curious if my small sample size is actually representative.