Sadly, I cynically think it's obvious which Mac line will be made in the US: the Mac Pro.
It's the smallest-volume product, and the one most likely to be customized. Therefore the one that needs the least-automated configuration. Since they already do final assembly on some CTO Macs in the US, it only makes sense for this product to be the one made here.
Hopefully that would be for a new line of Mac Pro's, perhaps with a revised form factor.