A quick look at the pages on the Apple Web site didn't seem to show anything until I scrolled to the bottom of the Tech Specs page. Lo and behold, there was a very short description of Bumpers. Available in six colors (white, black, blue, green, orange, or pink), Bumpers slip over the stainless steel edge of your iPhone 4. They come with metal buttons for volume and power and are made of a combo of rubber and molded plastic.
This may explain why we've received several tips over the last few days from people saying that iPhone cases are being removed from the Apple Stores. Bumpers are Apple's first foray into the iPhone case business (a market that has been run by third parties so far), and it will be fascinating to see not only how they sell, but what the reaction will be from long-time case manufacturers.