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.