According to Apple, “giving you free bumpers for that ‘antennagate’ issue that doesn’t exist had to end sometimes”. But, they didn’t say that. What they actually said, officially, is in the quote below:
We now know that the iPhone 4 antenna attenuation issue is 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. For everyone else, we are discontinuing the free case program on all iPhone 4s sold after September 30, 2010. We are also returning to our normal returns policy for all iPhone 4s sold after September 30. Users experiencing antenna issues should call AppleCare to request a free Bumper case.
So essentially since Apple doesn’t think this is really a problem, they’re ending the program. Which was expected anyways, and they’ll still give you one if you complain to AppleCare, but otherwise, Johnny Ive has some antenna designing to do, no matter what.
Source: Apple Bumper Case Program