Here’s the quote from Apple’s site:-
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.
You can get an iPhone 4 Bumper or case at no charge if you purchased an iPhone 4 before September 30, 2010, follow these steps to receive an iPhone 4 Bumper or a select third-party case from Apple at no charge.
1. Download the iPhone 4 Case Program app from the App Store.
2. Launch the app on your iPhone 4 and sign in using your iTunes Store account or Apple ID.
3.Select your Bumper or case.
For iPhone 4 purchases made before July 23, 2010, you must apply no later than August 22, 2010; otherwise, you must apply within 30 days of your iPhone 4 purchase. To qualify for this program, you must purchase your iPhone 4 by September 30, 2010.