A crack down the back of my iPhone 4 led me to look into purchasing a replacement.
The transparent back is delightfully geeky. Repair is as simple as can be. Two screws, slide the back off, slide the new back on, two screws. Done. Seeing the iPhone 4's guts through the transparent back is absolutely beautiful.
There is a small "sticker" on the back of the Apple iPhone 4 back, with a small contact spot on it. I had believed that this was part of the antenna assembly for the iPhone. (Not sure if it's for Bluetooth, Wi-Fi, 3G?) In iFixit's own words, "You can also see pressure contact on the inner face of the rear panel for what appears to be an antenna." I haven't noticed any issues so far, but I don't use Bluetooth and I'm rarely on the phone. . .