You can probably just contact tech support from Apple and tell them cracks started to appear on the back of your iPhone. They know this is a common issue with the new iphones so they will give you like two choices:
A. they send you a box to mail your iPhone back to them and they will replace the back.(Takes 2 weeks... so if you can live without an iphone for 2 weeks go for it)
B. they will set you up with a genius bar where you bought it and they will see for themself.