As you may or may not know there are a ton of theories on whether the iPhone 5 will be a completely new style or if it will just be a refresh of the internal processor to upgrade speed like the iPhone 3Gs was to the iPhone 3G.
I have no doubt that it will be a completely new design, and I don't have to go into all the theories of why (antenna issues among others), as others have already covered these (though these are a big part of the reason). The only reason I need is pretty simple and obvious. If it was not going to be a total redesign there would have been no reason for Apple to switch from its normal summer launch to the rumored fall launch.
If all you are doing is swapping out the processor, you would have no reason to push back your normal launch time. They pushed it back because they needed that extra time to get the new design ready as it was probably bumped up from a 2012 launch due to the issues with iPhone 4 and the changing (bigger screens and LTE) competitive landscape.
Do you agree with my assessment here?