If the cord starts to fray, Apple will replace it.

Regarding the extension cord, I tend to keep one in the bottom of my Tumi backpack because 1.) some outlets are very inconveniently placed or 2.) the brick won't fit into the allotted space.

It just sits there at the bottom of the bag and is hardly used.

I carry the brick and Europlug and U.S. plugs (my extension cord has a Europlug) in a Tumi electronic cord pouch (I have several that came with larger bags). Also in that pouch is a Morphie powerpack with a Gogo logo (thanks, Gogo!) and a lightning cable.