Don't replace it yourself. I tried that a few days ago and wrecked a perfectly functional iPhone 3G (earpiece speaker and proximity sensors no longer work). It was a total PITA because of the dozens of small screws and tiny parts. To replace the back, you need to remove everything, the screen, main board, battery, volume switch, power switch, vibrate/loud switch, camera, microphone port, vibrate motor, antenna, etc. It is NOT worth it.