Steve Jobs announced free bumpers for iPhone 4 at the Antennagate press conference. Apple enlisted top mass-market case designers to snag a free bumper from Bumper, Belkin, Griffin, Incase or Spek all the way through the end of September 2010. But incase you don't like this option, Apple made it easy to order free bumper "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," noted Apple on its site.
Here're the steps: Download iPhone 4 Case Program app from App Store; Launch app on your iPhone 4 and sign in using your iTunes Store account or Apple ID; Select your Bumper or case. That's it!