Jim Dalrymple reporting for the Loop:
According to Apple, 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.
It was quite clever of Apple to have iPhone 4 owners claim their free case through an application, no need to type in serial numbers, just login to your iTunes account or Apple ID and choose your case. Case choices include offerings from Belkin, Griffin, Incase, Spek, or Apple’s own Bumper.
Apple has also published details on their case program page for those seeking refunds for bumpers they have already purchased.