The money it would cost would be peanuts for them (a fraction of their bailout money ... and "Problem Bitcoin" would be solved before it's getting to big.
Sorry Frizz23 but it is ridiculous to assume that a commodity could be threatened by buying it. Will the gold market be destroyed if I buy all the gold in existence? Or the €? If I do that and destroy the commodity, I caused an explosion of its value. If I do that and dump all of my property afterwards, I raised attention and will get more people into the game than by not doing anything.
Even if you buy all but one Satoshi, the bitcoin would work for the world as it would now. We would only need to introduce more digits.
(Even one of the speakers at the Bitcoin Europe briefly sketched this "horror scenario" and I was only shaking my head. This is no threat, this would be the final break through.)