Is the model no. MD297HN ?
I think it is an issue with all iPhones of this model (and maybe other models too). Go closer to the router - it should connect. However, there is a problem and from what I have read on these forums - it is a hardware issue. Apparently, apple will replace if you ask. I have not yet done so - as I have been out of my country since I got the new iPhone 5. Will try once I am back.