We are talking about TSMC here. They have no problem building capacity to replace Samsung twice. Their problem would be that this is a huge investment, and they would want guarantees from Apple to make sure the investment pays. On the other hand, Apple has the cash to make those guarantees.
----------
That is always claimed, but I wouldn't think so. So Apple builds a new, faster ARM processor - well, who would have thought that? Of course they do. Samsung knows Apple will do that, whether Apple places an order with Samsung or keeps the thing top secret, because it is just common sense they will. Apple is looking for better batteries - well, who would have thought that?
Most things are just ordinary improvements that you would expect Apple to make anyway. Designs are legally protected, some technology is protected by patents, and both don't take long to copy - it's just risky because you can and will get sued for it. If Samsung wanted to make a phone that looks exactly like iPhone 5, buying a dozen on the first day it's released would give them enough time.