I have a white with no apparent cracks. However there is a small mfg defect near the volume button like a little bump about 1/16" long. I'm not upset about it (its a phone, not a piece of jewelry ya know). But when I look at it under magnification it appears to be an inclusion of some sort that is white, but has tiny cracks radiating out from parts of it. This is how I would expect stress cracks to appear and occur.
It's quite logical to me that these cracks are already present in the cases from manufacturing, but it takes a few days for them to accumulate enough dirt to appear dark and visible in the white plastic. Also logically, they are in the black phones as well but will never be seen.
So far I've only seen one pic of a case that was actually "broken". Although I suppose any of the cracks has the potential to propagate eventually, I would consider these to be generally a cosmetic problem, and a small one at that.
I would like to know if these cases are something more exotic than simple plastic. I wouldn't guess so tho. Apple is too cost conscious. I do think that they are beautiful, for being plastic. Oh, and under magnification, yes they do scratch. The are not made of any miracle material.