Are the people experiencing the 'cracks' in their iPhone 3G's using hard cases?
Lot's of places are still selling cases for iPhone (both hard and soft) but these cases were designed for the original iPhone with a flat back. Using a hard case designed for the original iPhone (assuming iPhone 3G fits in it) could cause structural intolerances to show up in the back plate of iPhone 3G, hence cracking at vulnerable points. Only use cases which state iPhone 3G on them.
Do not try and force iPhone 3G into the dock that came with the original iPhone. It does not fit. This would also allow structural intolerances to be displayed.
Based on the images posted by some users, their iPhones look very badly treated, especially the black iPhone 3G, what's all the damage around the SIM removal slot (the hole where the paper clip fits)? Looks more like somebody was trying to get the back off (wonder if that could cause the cracks?)
I am not defending Apple, I've had my fair share of run ins with Apple over the years regarding quality control, but I have a black iPhone 3G, which has been cased in a soft silicon case since coming out of the box. I have inspected it thoroughly with a very powerful Macro lens on a Canon 1Ds MkII under very good lighting. No cracks or scratches.
I place my iPhone 3G 8GB, in a dedicated pocket (yes all my trousers have at least 8 pockets) and treat it just like I have treated all my other mobile phones including the original iPhone. Use it regularly, since I got it on launch day, throughout the day, no problems with build quality so far.