How much protection do those $30 rubber bumpers that Apple sell for the iPhone 4 offer the device? According to iFixYouri, almost none at all.
If you're squeamish, you might want to look away ...
We’ve have known for a little while now that iPhone 4 bumpers do not offer all of the protection needed because customers have called in an told us. Although it does not take a video by iFixyouri to inform you that dropping your new iPhone 4 on concrete is a bad idea, hopefully this video squashed any doubts as to the bumpers effectiveness. A DIY fix for any type of cracked glass on an i4 screen can be difficult.
If the handset hits the ground side on, you might be lucky and end up with only some scuffs on your iPhone, but if it falls face down (and remember that the iPhone 4 has two glass faces), then it's gonna break.
Glass breaks when it hits concrete ... not really rocket science.