A few months ago, I dropped my iPhone 4 on a concrete path and shattered the glass coated back. I’ve dropped it countless times, and don’t use a case or bumper, so it was only a matter of time really.
I only discovered in the last week how simple it was to change the back though. There are 2 tiny phillips screws either side of the dock connecter port, and once those are loosened (you don’t need to unscrew them right out), you can press down on the back and slide it up about 5mm. The back then just lifts off. Probably best to turn your phone off first though.
I ordered a replacement back from ebay, and while I can’t be sure if it’s a genuine part or not, it looks good and works just fine! Doesn’t have the shabby chic aesthetic of broken glass, but a lot more comfortable in my pocket…