During the iPhone 4 press conference, Steve Jobs said that while the iPhone 4 antenna issue isn't affecting all users, they care about every user, and so will providing free cases to everyone who bought an iPhone 4 through September 30, 2010.

If you already bought an Apple bumper case, you'll get a refund. If not, Apple can't make enough bumpers so they'll source other cases.

The procedure seems to be, if you bought a qualifying iPhone 4, you'll get a website link to go to, pick a case, and Apple will send it to you.