There's been a lot of companies lately selling glowing Apple logo mods for the iPhone 4 and iPhone 4S. While they may look extremely cool and make your iPhone stand out from the rest, you should also take into consideration how these work and the potential risk they pose to the functionality of your device.
First we will state the most obvious - it will completely void your Apple warranty. Opening your device in any unauthorized way will void your factory warranty with Apple. So if your device is still in warranty, I'd stay away from this.
