With two sides of glass rather than one, it seems that you are going to have about double the incidents of breaking glass on the iPhone 4 as you would on previous generations of iPhone which had plastic backs.

Also, the plastic around the sides of the iPhone 3GS cushions the impact significantly more than the iPhone 4’s metal antenna-frame.

Highlights of the study include:

--  iPhone 4 owners reported 82 percent more damaged screens in the first
    four months compared to iPhone 3GS owners
--  More than 25 percent of damage reported was to the glass on the back
    of the unit
--  The reported accident rate for the iPhone 4 was 68 percent higher than
    for the iPhone 3GS
--  An estimated 15.5 percent of iPhone 4 owners will have an accident
    within a year of buying their phone
--  iPhone 4 screen damage is responsible for more than four-fifths of reported 
    accidents during thelast four months, slightly higher than the iPhone 3GS during
    the same time period.

SquareTrade is the largest independent warranty provider in the world.  They analyzed iPhone accidents for over 20,000 iPhone 4s and the results largely lined up with their earlier predictions.

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