I have a Graphite K3 and a Graphite DX. Both have developed hairline cracks but in different locations. The K3 cracks began simultaneously in both lower corners and gradually "grew" over time, extending all the way to the edges. The DX crack is a single half-inch horizontal hairline crack on the upper right, directly abeam the Home button. All of my Kindles, including previous generations, have been handled with extreme care, never dropped, not a scratch and look like they are brand new (covers too!).
Before the warranty expired on the DX I called Amazon and they sent a replacement. I was expecting a refurb but it arrived scratched in numerous places and was rather beat up. I was so peeved about the unacceptable condition that I sent it back and kept the original with the crack. This was the only time I have been unhappy with Amazon's customer service. I should not be required to return multiple replacements in order to have a Kindle in a condition at least the same, or better than, the one being replaced. Especially for the price of a DX.
I am not going to make the same mistake with my K3.