There are too many black case-less iPhone 4S' floating around so finding a way to distinguish one phone from another without adding a bulky case and not compromise the aesthetics of our phones was assumed to be a challenge. Luckily, Monolith makes incredible replacement backs for iPhone 4's and 4S'.
The installation takes all of a couple minutes and the end result is about as perfect as any person could want a polished wood panel for a mobile device to be. Plus the case doubles as storage for your current glass back panel; just in case you need to take your phone to the Apple Store and lie about not modifying your phone ever.
There are a ton of great options so if anyone around your office wants to copy your style, hopefully they will at least choose a different wood panel than you did.
More styles available at Monolith.