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.
Thanks for the answers, to diverge again as to why: If bitcoin continues unchecked it will likely displace fiat currency to a large degree. Fiat currency is the main power source for the entire banking industry and to a lesser degree the US Govt. The US / European Establishment are very unlikely to stand by as a large portion of their power structure is dismantled and handed over to diffuse individuals. You can see already with the Mt Gox account seizure, bank account closures, etc. what the govt/banks think of bitcoin - they want to destroy it. They have already done the same with EGold, Liberty Dollar, and many others. However, bitcoin is so widely held, popular, apparently difficult to attack, and clearly a moral good that a traceable attack, at this point, is going to be a last resort. I'd expect to first see a continued propaganda effort and further attacks on the fiat-bitcoin interface. This will drive bitcoin further underground and give impetus to the propaganda that it is the currency of illegal activities. Eventually this can attempt to build public support for destroying bitcoin. So, back to the most important questions...
at this point I cant imagine Bitcoin taking over the Fiat system, I might change my point of view when larger adoption appears or when governments make an official statement about the hole thing, but you have no idea of how governments are struggling to change their Falling fiat systems, they know the size of the mess they are dealing with but change is not that simple as we think.
many will disagree about what I will say, but I truly think that regulation is a really important thing here, which bring us back to "technical part" of an attack.
the theory of a 51% attack is possible, the motive is the question again. so lets say, to get a 51% of the network you need around $500 million worth of equipment add to that electricity AC,internet..... that money will be worth of using only once because when you take Bitcoin down you just achieved something and you lost $500M, but again this doesnt mean the death of Bitcoin.
if you both have $50 billion, would you invest $500 million to "try" to kill a supposed threat, or wouldn't be easier to take the ride and invest and double/triple/... your money.
I understand you input here, I just want to convince my self why anyone will try to kill Bitcoin. because you know if they wanted to do it, they could do it long ago when people used to mine with CPU then GPU. which again bring another question, if governments and banks hate bitcoin, why didn't they compromise the network when it was smaller and weaker ?