I wrote a longer answer to the 51% network attack over on r/bitcoin. The way I see it, the attack is almost trivially easy to pull of for a large motivated entity, such as, say, the US government.
There are two layers of protection against this happening, imo:
one, at the moment the price is low (300 to 400M USD), but *interest* to attack btc directly is also comparably low. That might chance as btc becomes more established, but then the *price* will also be higher.
two, network composition. if any large group suddenly enters the scene, outcomputing everyone else, that will most likely be noticed, and investigated by the community. I don't believe it would be easy, even for a government with loads of money, to pretend to be several thousand individuals forming a mining pool.
300 to 400 MILLION dollars is extremely expensive for anyone, that amount of money needs to be accounted for, and, preferably, not wasted. It's possible that someone could do it, sure, but there's no crime without a motive, right? Also, considering how much hash power that would bring you, only a crazy person wouldn't just decide to be 'la central bank' of bitcoin. Just too much money required. Besides, if they got started with their project right now, considering how fast the hash rate is going up, I wouldn't be surprised if it turned out to be far more expensive to do by they time they finished then they expected.
