As has previously been posted, Apple's description of the bumper mentions nothing about any protection:
"Made of durable rubber and molded plastic, the two-tone iPhone 4 Bumper fits snugly around the edges of your iPhone 4 and gives it a splash of color. It's a fun and unique way to personalize your iPhone 4."
"Dress up your iPhone 4 with an iPhone 4 Bumper. Choose one of six colors white, black, blue, green, orange, or pink and slip it around the edge of your iPhone 4. With metal buttons for volume and power, two-tone colors, and a combination of rubber and molded plastic, Bumpers add a touch of style to any iPhone 4."