I want this iPhone 4 back. It is not a case. It is not stick on. You actually have to remove the glass back to replace it with this. Check out this link to see how easy it is. Add a Metal Back to Your iPhone 4
If you like some other material, or color or any other spare parts like the now not included SIM ejector or sim tray etc … check out this site at cnn.cn. It is damn cool for the geeky DIY. But will see if i can find shops in HK.