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I just bought an iPhone 4, and like most I had to spend an extra $29.99 on a bumper. However the bumper hasnt help the signal problem much (it helps a little), but its not a solution. The bigger problem is the opening on the bumper for the Dock connector. It works fine with the included usb cable, but it doesnt work with most 3rd party or even old apple cables. Case in point the apple Dock to VGA adapter, it works fine with no bumper. But, add the bumper and the adapter doesnt lock into the Dock connector, so it doesnt work. The simple fact is the cutout on the bottom of the bumper isnt big enough for any cable but the apple supplied one. Save your money and try this. http://www.engadget.com/2010/07/01/iphone-4-gets-a-1-alternative-to-pricey-bumpers/
Mac Pro Mac OS X (10.6.4)