A few weeks (July 30th) ago there was a real scare about iPhone 3G’s backs developing small cracks. Most of the reported incidents were white 16GB iPhone 3Gs. Although a few black iPhone’s developed cracks also. From reading/posting on numerous forums on iPhone’s backs cracking, most people came to agree that it was a large batch of “sour” iPhones. Most threads about this topic are long inactive, so if you are looking into an iPhone 3G, don’t be too concerned about the iPhone 3G cracking issue.
