Having US manufacturing plants in the current economy is viable. People is willing to work instead of staying unemployed (yes there are some individuals that like to collect unemployment for a long time, every county has them).
How many of you actually work or had worked in a manufacturing plant?
The manufacturing cost will go up, but the quality will go up also, and could have avoided the recent (last 5 years) quality problems that we had and are currently experiencing with

products. Why I write this? Because I am sure that American manufacturing workforce will be better focused on preventing issues and actually solving them before shipping the products.
The 3rd party OEM - Apple relationship model allows them to produce and sell all what they can and it is very easy for Apple to simply replace a defective product like an iPhone when an unhappy customer shows up at a genius bar, and the unhappy customer many time receives a product that still may have the same codition. Apple charges back the OEM supplier and they don't loose anything but sometimes even make a gain in their perceived super customer service instead.
The balance between paying more to manufacture and the reduction in quality issues will make a difference.
The current and future apple employees will happily work at home instead of continuously traveling to this so called 'slave cities' and will be better focused on producing, designing, testing and improving the products. I think a good model will be to have a lead plant in the us for every product and once the design and manufacturing process us mature enough they can move the production to the sister Asian sites. That way they can have a balance between faster concept to sales cycle. I believe that all the manufacturing processes and new technologies are developed by apple employees from the US, as well as the supply chain management is done by apple