The iPhone 4 bumper, I hear, is pretty good at keeping the iPhone 4 signal from dropping even when you try the ill-famed death grip. That does not actually mean it will help you protect your iPhone from drops.
Folks at iFixYouri educate us about this the hard way using their drop test. In the test, a video of which is embedded below, the iPhone 4 with the bumper on, is dropped from normal standing height on to a concrete floor. The iPhone glass case cracks on the third drop – very much like it does when it is unprotected.
The iPhone repair company however explains that the number of drops before the glass cracks depends very much on the side that hits the concrete first.
“What really matters is how you drop it. If it hits on a corner you may get out of trouble completely, or with just a scuff mark. If it drops on the glass though, on top of concrete, you could be in serious trouble. This scenario is very likely every time you get out of your car.
We also performed an iPhone 3G and iPhone 3G test with protective cases and instead of the glass breaking, there was LCD damage on the fifth and sixth drop, respectively.”
I would not advise you to try this on your iPhone 4 for confirmation. But anyone doing so is welcome to share their results