Sure, you could add a faux leather skin to re-create the classic Leica feel but why do that when you could just print out a picture of a Leica Camera and tape it to the back of your iPhone 4?
A MacRumors user printed out that picture of a Leica M8 and placed it inside a Griffin Reveal Case to create his mock Leicaphone. He cut out holes for the camera and LED and blacked out some white spots to make the picture an exact fit for the iPhone 4. If you want to get started on your own, he recommends you print out the picture of the M8 to 2.282" x 4.528" at 600 dpi. [MacRumors via Unplggd]