I'm in engineering plastic (injection molding) business. The molding defects (hairline crack) in our industry is a pretty common one called weld line.
http://en.wikipedia.org/wiki/Injecti...olding_defects
http://www.plastictroubleshooter.com...knit_lines.htm
It happened to my older generation DX (white) too right in between the USB port at the bottom and the space bar. It was probably there in the first place invisible, probably at where the molten plastic flow meet around the hole or the obstacle in the mold (space bar and USB port in my case), making it one of the weakest spot in the molded part and it probably started to crack open over the time with slight pressure applied. Given how thin the plastic housing of the Kindle is I'm not at all surprised this happened.
Now does anyone know if replacement policy is applicable for overseas consumers? And if upgrade with Graphite model for extra cost available?