It's possible, but there is the time difference thing, especially when it comes to the east coast. We have 6 hours to the east coast, 9 to the west coast. In Spain people like to start and finish late (having dinner at 9pm is normal!) but this difference is uncomfortably much. It's not really handy if you have to work US time, bring your kids to work, have a social life, etc. In my current job we're the "inbetweeners" between the India and US teams. We keep in touch with both because India and the US don't normally collaborate much due to an even worse timezone situation.

Also, why would they bother paying US salaries to employees stationed in Spain? When I moved here from Ireland (also EU) my salary actually went down a bit :) And they already scaled me up a lot within my salary class, if I had come in here 'off the street' I would have made even less. Multinationals already have this scenario a lot and they pay local wages.

People are willing to work here for local wages. And local conditions are mandatory here if you live here. Working for a US company and for US conditions while living here will not fly by EU law (the only exception is NGOs like the UN).

So I don't think it will change too much to be honest. If anything it will provide more downward pressure on US salaries than upward pressure on the ones here.