iFixit released transparent rear panels for the iPhone 4 several months ago, which put the surprisingly attractive innards of Apple's smartphone on display. Apple must have tweaked the internals of the iPhone 4S just barely enough for that panel to be incompatible, because iFixit has now released a new rear panel specifically engineered for the iPhone 4S.
For a limited time at a price of US$29.95, iFixit is selling a kit that includes the rear panel, a "liberation kit" that includes the special "pentalobe" screwdriver necessary to remove the existing panel, a screen protector, and a "thirsty bag" that will dry out your phone if it has an unfortunate encounter with liquid. iFixit will continue to sell the back panel by itself for the same $29.95 once the promotion ends.
Swapping out the rear panel on an iPhone 4 or iPhone 4S is incredibly easy once you have the proper tools at hand; I swapped out the panel on my old iPhone 4 in less than five minutes earlier this year. If you've cracked your iPhone's rear panel and are looking for a somewhat quirky replacement, iFixit's transparent panel might be what you need.