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
necro
The sum total hashing power of every supercomputer in the world is much less than the total hashrate of bitcoin. Supercomputer almost always run computations that are rely heavily on CPU, memory, and I/O resources, but these are not suitable or optimal for mining bitcoins at all. Not only was this case true just with GPU miners contributing to the bitcoin network hashrate, but now that we have ASICs...
Most supercomputers are built for performing general computing tasks that require huge resources because of their inherent complexity. The processors must be able to perform a extremely wide range of operations. But in bitcoin this is completely not the case... we need high speed hashing of SHA-256 algorithm... ie. the same few operations over and over as fast as possible and running in parallel as many times as the hardware allows for. The hardware in each case are almost like polar opposites.