I am now using a 3rd 14.6GB black iPhone "3G".
1st one had the plastic crack.
2nd one was badly repaired with flushness between the silver bezel and the plastic case + burs on the silver bezel.
3rd one: noticeable GAP between the silver bezel and the plastic case.
What

is doing is recycling all the returned iPhones between us.
And sadly they poorly repair them.
My Theory is:

has a significant dimensional variation with this plastic case production, due to this dimensional variations the plastic case is forced to fit and the cracks appear later on.
Only

quality department knows which cavities, where they inject the plastic cases, generate this variation. I hope they identified this and generate a clean point soon.
Only after this happens and when they supply good replacement units all over the different

stores, we will be able to obtain good replacement units.
Also if they don't stop recycling the returned phones,

will keep passing the bad phones between us.
After a 2nd replacement we deserve a new flawless unit in replacement of the defective one.