Tim Cook serves up a lot of Kool Aid when it comes to this topic.
What a lot of companies have found out, is that bringing back assembly to the US has resulted in more efficiency and a lot more worker collaboration with figuring out ways to optimize assembly and even design.
True, you can't go around waking up American workers at midnight when Apple can't seem to get their design finished in time. But that's a rare occurrence. What's more important is being able to ramp up production at a sales launch each year. This kind of seasonal work is already quite common in the US, with hundreds of thousands of workers especially moving around the Christmas season to work at places like Amazon and UPS.
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Back to Tim, he once made that famous statement that all the tool and die makers in the US would fit in the meeting room (which held about 5,000). The naive assumed this meant there were not enough to go around. Which is of course what he wanted people to think.
What he meant was that there's now about 5,000 COMPANIES in the US, not just that many machinists. Moreover, you likely need only need one company to create what's needed for a factory.
Again, Motorola via Flextronics already proved you could make phones (and customized ones at that!!!) in the US, for only about $10 more per phone.
Interestingly, Apple also uses Flextronics for their US Mac assembly plant.