I think 51% can be done quite easy for the U.S. government: they need special squads/agents to take over of 2 or 3 biggest mining pools. This can be done even if pools are operated outside of the U.S., and even can be done remotely. But i guess, to have more confidence they would need to send special people to pool operators (-; Hell, at the moment we cannot be sure that some of the pools are already in control of governments....
This would only work for as long as the pool miners continued to run their workers. As soon as word of the takeover got around, miners would stop using these pools and Bitcoin could get back to normal operations.
But the US does have some serious supercomputing hardware at its disposal. I wouldn't be surprised if they could take over Bitcoin quite easily at any moment.
-Michael