Apple says that 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.
I find it a pretty cool idea to launch an iPhone app to handle the free Bumper program. The app is basically a limited edition of the recently launched Apple Store for iPhone app.
You can download the iPhone 4 Case Program here. See also the iPhone 4 Case Program page on the Apple site.