I mentioned it elsewhere earlier in this thread,but according to a Nintendo rep I spoke with it was caused by the hinge being tightened up beyond the proper torque spec. A batch simply were screwed down too tight.
I don't have proof from God of this or anything, but some of the speculative theories in this thread of what's been causing the cracks are ridiculous. Fer instance - just a couple posts back someone insinuated they used cheaper plastic in the first USA batch than in the Japanese models.
When you squeeze something hard enough, if it cannot viscously flow around the pressure, then it will crack.