I don't baby my poly-unibody and I routinely carry it from the corner when the screen is open. No cracks that I can see thus far.
yeah i think these people with cracks either arent telling us something or had an unknown mishap. the first poster also has a crack near the ethernet port, which to my eyes seems impossible to crack unless it has been dropped.
anything plastic can crack if dropped or forced beyond its bending point. thats like buying a glass piece and expecting it not to break when you drop it...