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.
If your iPhone is out of warranty, there's still concerns to consider. Accelerated battery drain will be an issue. While many of these companies claim there is minimal draw on power, there is still extra power being consumed. An extra light source will always pull power. So yes, you will see more battery drain than without the modification installed.
My biggest concern is how these light mods actually function. They use the existing circuitry in your iPhone to draw power, specifically the LCD cable. This is where the glowing panel pulls power from. Not through the battery but by pulling power through a connector that is already being utilized for something else. In this case, your screen. There have already been reports of washed out LCDs due to glowing mods.
I also don't recommend putting stray components in your device that aren't held down in an appropriate fashion. Your device is designed to pull the correct amount of power from the battery. Your logic board is also designed to handle hardware a specific way. In a worst case scenario, you could potentially short out your logic board.
There's too much room for error with these types of products and not enough information on them. Not to mention the error that can take place if you don't have any idea what you're doing when installing these modifications. You could end up breaking another component in the process, or even worse, break a connector off the logic board. Until a modification can be created that pulls power directly from the battery opposed to using existing circuitry, I won't be giving my thumbs up.