Ghash and BTCGUILD has a total of more than 51% hashpower.
The government just need to hack these two pools.
They can hack it with a 0day (a vulnerability that is only known to them and not fixed)
this theory is the same as government having the ability to hack any server they want. if they can hack ghash and btcguid, they can hack other pools as well. NSA can already decrypt SSL, they could have cause massive chaos if they want. they could even hack bitcointalk.org and all asic miner's website.
Snowden described a formative experience the situation where they setup an elaborate situation to make a swiss banker drive drunk and be caught, so they could recruit him as an informant that he could deliver secret banking information (which is a big deal in switzerland) to the US government.
The leaked documents show that they have infiltrated a plethora of corporations to introduce bugs into common security standards as well as ensure that weak encryption standards are chosen by international panels (ex, they helped ensure that the weakest of the 3 finalist competing standards was chosen for AES).
I can definitely see them doing something hamfisted or sneaky to the people who own BTCGuild and Ghash. Unlike P2Pool those are centralized guilds