most of the cracks show up weeks/months after purchase and are allways in the same spots:
headphone port area, volume/silent buttons, camera, and charging port/bottom... It is still unclear if this is a defect or design flaw...
They are not breaking on their own, but are definitely not the user's fault, as many are cracking under normal use...
If the cracks are your only problem be greatful, and look over the replacement before you leave the store, as I've been given replacements with chipped chrome headphone port, chips where chrome meets the back, multiple bright pixels, that are hard to see in the store, but noticable in low lit rooms, broken proximity sensors, screen failures, battery problems, light leaking from the left side(which apple has refused to recognize as a problem...etc...
So if you have cracks in these areas, and havent dropped or sat on your phone, bring it in, the geniuses have been great every time I've gone in...