iPhone crack in rear casing near dock connector

I have looked after my iPhone (without a case though, I must admit) and noticed two small cracks which have seemed to come out of nowhere, one at the side of the dock connector and one growing right from the middle of it. I have heard its a common problem with iPhone 3G/3GS! My iPhone warranty runs out in April 2011, but I no longer have any proof of purchase. Is it likely that Apple would replace the iPhone, or even just the rear casing? I have the original packaging/box just no receipt. My nearest Apple store is about 2 hours away, I dont want to get all the way there to find out they wont cover it! Thanks in advance.
iPhone 3GS 16GB (white) iOS 4 iPhone 3GS, iMac 21.5"