While I'm still waiting for an inexpensive wood case replacement for the iPhone 4, there's a solution for a mere $US25: Karvt's iPhone 4 skins crafted in real wood. I can't decide between chocolate walnut and natural cherry.
The pine, cherry and bamboo versions look beautiful. You can get them in maple and walnut too. I wouldn't use the front skin, just the back. And only until a true back replacement came out. [Karvt]