If large Govts bought all available bitcoins in an effort to wreck BTC it would be a drop in the bucket for them. ( and the Fed would only need to print the $ to do it) They would also be in a position to manipulate the market.
Furthermore if they embarked on mining using their massive resources it would probably push ALL small miners out of business.
Therefore BTC can not be regarded as a serious threat to their currency monopoly, merely an interesting diversion for the powers that rule the markets
Then whatever government does it, is a total sucker.
Bitcoins empty? Damn, some people got rich. Lets mine Litecoin, namecoin, novacoin, feathercoin, yourmommacoin.
If all specs are provided, you could probably inplement a new coin in one day. Then they have to buy again.
And again, and again....
Until people are laughing their asses of, because Cryptocurrencies then become something awesome: A get rich quick scheme that is BACKED BY THE GOVERNMENT because they will not bail but buyout.
What could be cooler than that?