mining these days is mostly centralized, you have few pools that have the most of the network hash power, p2pool is not that popular which really bothers me, so an attacker (could be US government or any one else) in has to achieve that 51% in order to start his own fork and do a big mess.
the easiest way and most efficient is to heavily DDOS the biggest pools, they say that they are DDOS protected but 100 DDOS protection is a myth, so lets say the attacker can take the biggest 3 pools (BTCguild, GHash.IO, Eligius) this is equal to 66% of the network hash rate.
so the technical aspect of the scenario is possible, but the question that popup is why ? if I have that kind of power I would just mine and make money, because such operation will cost millions of dollars, who wants to invest (lose) millions of dollars to do such thing ?
the million dollar question is what the government and big organizations think of Bitcoin ? in which position Bitcoin stands ? is bitcoin a threat for them ? will they ever regulate bitcoin ?
it is a serious matter that should be discussed but when you have the answers to these questions it will be easier to imagine the 51% attack.
The biggest problem is the mistake that was made not to allow normal computer processors to mine efficiently. If this was made, then millions of people would have their miners running. With millions of users mining, you could never easily take over 51% of the market. But instead, the mining is designed in such a way that you can develop specific cards that make normal computer invalid. What happens then? Well, all the mining is centralized to those few who have these cards. And it's much easier to target.
