Although I have not heard anything about this recently I guess the pending merger of American Airlines and US Air has brought this debate back up. Most major airlines currently outsource their heavy maintenance. While this is nothing new, think TRAMCO and the like, the newer trend is outsourcing to companies that are not in the USA.
This is a trend that I do not think will slow down or stop anytime soon. Of course the unions are up in arms over this (who can blame them) but the truth is that personnel is one of the highest costs for any airline.
There is the nagging question of regulation at these MROs that are outside the country but I feel that they are being addressed. The FAA requires the MROs to operate with similar regulations as any MRO inside the states-but-who is doing the regulating?
I don't think that any airline would get rid of ALL their maintenance to go the full foreign MRO and contract MX route. They know that NO ONE would take care of the planes as well as their own employees.
Anyway check out this link to read the article that brought all this up in my mind:
Congress Wary of Outsourcing Aircraft Maintenance-Roll Call Policy