I have read other accounts of similar cracking. Most (all???) have been in the graphite colored one.
after reading this thread I checked out my Kindle 3 for hairline cracks - I have a Graphite coloured one. No fractures
It freaked me out a bit when I managed to download some mp3 to play. Techno geek I am not, but I decided to play and explore all of the K3's abilities. I plugged in my headphones and no audio!! then it went into sleep mode!!
Thankfully when I did a restart everything returned to normal.
However it has every so often, just recently started to either; turn back on after I have turned it off ;or do a re-start (the amazon/tree silhouette logo screen comes on). And it kind of creaks where the back seams meet at the top between the speakers.
At this point I would be lost without it, so I am just hoping it is just one of those things.