A 51% attack is about the dumbest thing someone could do. If you have over 51% of hashing power surely it would benefit you more to mine BTC as opposed to trying to double spend whatever BTC you currently have.
Of course a national govt could want to do this but there are easier ways to disrupt BTC (for a national govt).
It is important to break the "51% attack" into two groups. The economic attack (engaging in an attack with the intent to profit from the double spends or possibly by shorting BTC) and the destructive attack (engaging in a 51% attack with the end goal of destroying or significantly marginalizing bitcoin).
There is little value in an economic 51% attack. The cost of such an attack far outweighs the amount of commerce which could be double spent. Add to it the difficulty in having funds/goods delivered without detection and it really is just a monster under the bed ("don't let the 51% man get you, he will make your coins disapear".
The destructive attack however if still viable for a major corporation, a cartel of such corporation, or a governmental entity. While those entities do have other resources (legislation, internet filtering, FUD campaigns, etc) the 51% attack can be done covertly. Major governments engage in covert operations all the time. Billions upon billions spent on programs and projects that the tiniest fraction of the citizenry even know exists.
That being said do I think a govt is going to 51% attack bitcoin? No. It likely will be futile. The destruction on Napster only caused the community to develop even hard to attack resources and protocols. In hindsight it would have been far better for the media industry to work with napster and develop a way to monetize it.
