Hi,
My kindle (UK 3G model) also developed a hairline crack on the bottom right corner:
http://img811.imageshack.us/i/img2299n.jpg/
I wonder if this common problem is due to the way we handle it: in my case most of the time I hold the kindle on the bottom right (I am right handed) and balance it between my thumb and index finger, right about where the crack appeared.
Even though it didn't bother me too much, I decided to replace it so that Amazon doesn't get away so easily with that flaw.
To their credit the replacement procedure in the UK is very efficient: initiate a support call from amazon's website, explain the problem and the next day a replacement should turn up via DHL. Then just post back the old kindle in the new kindle's box with a printed pre-paid stamp label.