We now know that the iPhone 4 antenna attenuation issue was even smaller than we originally thought. A small percentage of iPhone 4 users need a case, and we want to continue providing them a Bumper case for free.
Apple didn't quantify the "smaller than we originally thought" or "small percentage" with hard numbers
The free case program will be discontinued for all iPhone 4s sold after September 30. The company will also return to its normal return policy for iPhone 4 after that date.