Well, I have read this thread and can help to answer some questions, which some of you guys may have already know. Anyways, all iPhone 3G are assembled by Foxconn (a Taiwanese company) in Shen Zhen, China. They are the world's largest EMS (Electronic Manufacture Services) with revenue of 40 billion USD, so not a small company even by US standard. They have a business group that is dedicated to Apple and pretty much makes all Apple products. Foxconn also makes Dell computers, Cisco routers, Nokia phones, Moto phones, you name it and they pretty much have a hand in it.
About this crack, yes, it could be that it was a bad batch, but Apple is notoriously for demand products to be made by their appointed vendors, so it could also be Apple's fault. Whatever the case I am sure Apple and Foxconn will find out the reason and do a running change.
First gen. phones are made with higher quality materials, that is always the case in this business, of course, they could also have other issues because at RD phase, something always gets lost, only in real world testing by actual consumers that problems will start to surface. Products made at latter stage will iron out those design issues but will be made with more second source materials, especially the EE components (but not CPU themselves, they usually have no second source).
So depends on how you look at it, Apple and Foxconn will eventually fix this crack issue, but those phones will likely to come with additonal second source materials as well. Regardless, 3G is the second gen., which means compare to the 1st gen., most EE/mechanical parts would have been replaced by cheaper components already.
After ramping up, cost down is the number one priority for EMS and buyer, after all, that is how they make money (especially for EMS). They will try to put in an inferior parts whenever they can get away with it.
