If I would be a nation state opposed to bitcoin and use cyber capabilities against it, I wouldn't run DDoS attacks. I would hack MtGox and steal their coins. Both China and the US should have the capabilities to do so. If all the bitcoins of the exchanges would be constantly stolen.
They keep most of their coins in cold storage, so unless they're going to pull some physical black ops stuff, there's no stealing the bulk of any exchanges coins. Not to mention, I doubt any expensive 0day exploit most governments have access to would not risk loosing that asset against a system that could possibly reverse engineer and ruin the very expensive "super secret 0day".
The botnet's that run the DDoS on MtGox are probably payed via bitcoin by someone who spectulates on the price changes that the DDoS attacks cause.
DDoS attacks are cheap. You don't need to be a nation state to run a DDoS. You just need to pay a small sum of money to rent a botnet.
But back to a nation state attacking bitcoin, cyber attacks aren't needed. The US could simply start suing all bitcoin miners for facilitating money laundering and violating the principle of know-your-customer.
Announcing such law suits would alone do a lot more damage to bitcoins price than the DDoS attacks against exchanges. If current laws aren't enough congress can always pass additional laws.
The world doesn't look the way it would if the US government would make an attempt to shut down bitcoin. The US government isn't filled with the smartest people but if those people would go after bitcoin they could still do serious damage. Damages that exceed the damage that a single individual can cause by rented a botnet with his bitcoins.
Governments operate on the theory of plausible denial ability, so no one is going to see any "proof".
Actually the US is perfectly able to attack other groups without any plausible denial ability.
The Government would much rather keep things covert until the point where the gloves must come off and then they start saber rattling, followed by making laws, and examples of people to really shut Bitcoin down. People should really check out the guy that wrote that article. He claimed after the "correction" that this was just "their" first attempt to crash the thing and were most likely gauging the market to tune their software. He claims that when they really want to bring it down (and roll people in front of the cameras) is when the real carnage is going to ensue. He says they might not use their software again for years, or it could be next week, should the market go parabolic again or TPTB feel threatened.
http://www.naturalnews.com/039850_bitcoin_bubble_crash_globalists.html