After shattering my iPhone the day I got it, I called up Apple to make an appointment at their Genius Bar to see what they could do. Over the phone, they weren't of much help.
The earliest appointment they had at my local store was for a week in the future. I decided to test my luck and showed up at the store anyway to see what they could do for me in person. A Genius—who, at the time was working on two other people's phones who complained of poor battery life— took my phone behind the counter for inspection without saying much. After a short while, he came back and asked me all about how the back panel had broken. At this point I perked up because I had heard the anecdotes of people having their broken phones replaced by Apple recently.
I told him every gory detail of the phone's demise. He put in all of the data and came back telling me I was in luck. Apple takes back the first 50 phones with physical damage to assess what went wrong and improve their future models. I happened to have been lucky number 49 in the country to bring my battered phone back to an Apple store for inspection, so they gave me a brand new replacement phone on the spot, free of cost. I turned out to be the fourth person from that store alone to have their phone replaced during the way. Two other back panels were brought to the store and sent back, as well as one front touch screen panel.
Apple was also replacing iPhones that have a defect in the speakerphone functionality. Apparently the secondary mic (located on the top of the iPhone next to the headphone jack, and shaped like a pin hole) was causing issues with the speakerphone. However, they didn't mention how many of those phones they would be bring back to the factory for inspection.