I will be taking my 3G back to the Apple store for a replacement due to hairline cracks. It will be my third 3G, as the first one developed cracks in the typical places, and the second one developed some pretty bad cracks since I got it two weeks ago. (the lock button is also defective, since half of it sits flush with the housing) I have the black model, so they're not really noticeable, but they bother me regardless. (I'm not paying $200 bucks for a device that can't even last two weeks without some damage) I baby my phones; every 3G I've had has spent almost all of its time in a case, have never been dropped, and are never put in a place where significant pressure is exerted on them.
I do believe, however, that the 3G has a design defect in the housing by the volume rocker. In the latest model I have, the most serious concentration of cracks (seriously, we're talking at least 40 hairline fractures in a little more than half an inch) is between the housing and the rocker. I was rubbing the housing with my finger nail to see if I could feel them, and noticed that that particular piece of housing is able to be bent in towards the interior of the phone. Because there is nothing behind it, and because the housing is thin, that pressure causes cracks to form. (just like you would expect with anything with a coating over injection molded housing like the 3G has, even though the actual plastic is not cracking) Even if you're not trying, the pressure required to activate the rocker would cause cracks to form. There's no way around it, short of redesigning the housing.
It's too bad too, because the 3G has generally been the best phone I have ever owned. Seriously, I would give up the HSDPA radio in order to get the 2G's metal backing. Sure, it causes reception issues, but it is also incredibly durable. Hopefully the next iPhone, even if it retains the plastic back, will have a better executed version of it.
