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My 3Gs has almost the same crack in it. Apple will most likely replace it.

Personal note: I'm not replacing mine, as I find the crack gives it some nice qualities.
 
So I got massive crack on the back of my iPhone 3G reaching from the charger port up to text on the back. I heard that this is considered a hardware fault and that Apple will replace it for me free of charge, is that correct?

Below is not my iPhone but demonstrates pretty much my issue...

http://www.pocketup.com/images/iphone3g/black_iphone_crack1.jpg

My 3g had the same issue. You probably have cracks by the silent/vibrate button as well
 
Ha! Actually I do...and by the headphone jack as well, I'm definitely having this fixed!

I had the same exact issue and Apple replaced it. Just make sure: you're still under warranty, and there is no water damage (they checked mine.) Also, make sure you back it up before you go to the Apple store. They'll have you wipe it right on the spot.
 
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