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In a message posted to its website on Friday, the company says "We now know that the iPhone 4 antenna attenuation issue is even smaller than we originally thought."
Those qualifying for the iPhone 4 Case Program can currently use an app [iTunes link] to request a bumper. The iPhone must have been purchased after July 23 to acquire a case through the app. After September 30, it seems that the iPhone 4 Case app will be discontinued for new purchases.
If you plan on purchasing an iPhone 4 after September 30, you can still get a free bumper — you just have to call AppleCare.