I would like to see anyone try to take on such a huge network. A bank couldn't afford to do it. Perhaps with the NSA's budget they may have some crypto chip that could outhash anything out there. Otherwise I simply don't see how to take over 51%.
Right now the bitcoin network is using 58.710 T/hashes. Everything is gauged in Mhashes so that is 58 billion 710 million M hashes. Top GPU does 1200 Mhash so you'd need about 49 million in GPUs of them running. At $777 each that's 38 billion to duplicate the network, doubling it. Not including anything else on a computer.. no boards, no CPUs, no electric to run them. I think my math is right.
Total value of all bitcoins if bought outright is 10,936,000 BTC * $72 = 787 million. Sure an entity could try to buy every coin out there. That'd be interesting to see if that would shoot the price even higher. I'm not sure how that would play out. But BTC can go 8 decimal places so even if they get very expensive people can use small portions of them.
Shutting down the peer to peer network is about the best that would be doable. Outlaw bitcoin, shut off anyone using it, and perhaps. But there is an estimated $2 million USD done on the silk road every month so people are fine doing illegal things.
This is all about control. Bitcoin was created to avoid the banks. There is no way to "print" more USD to buy treasury bonds like we're doing now. The banks with direct ties with the government is probably fearful this idea of being able to avoid banks entirely actually catches on. That would not be good for business.