Apple will end the free cases program would end on September 30th.
Apple has started the free iPhone 4 bumper program only if you buy the smartphone before September 10. 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 take this program, you will have to download the app from the App Store to start the process.
More info: iPhone 4 Case Program