I'm actually a strong believer in bitcoin don't get me wrong.
And this question has poped into my head sometime in the past but I never paid attention to it at all.
But when someone that just heard of Bitcoin mentions it, it kinda makes you ponder on it a little more than when you think of it yourself.

You can't deny however that for someone who has tons of fiat to throw around, this is way easier than the 51% attack we hear so much about (yes, this is a different kind of attack - but still effective enough to do serious harm)

I think the good thing is the fact that Bitcoin being mined slowly for the next few decades is one of the best defenses we have againt this kind of attack. At least we would have time to organize a response. If at a certain time a large portion of Bitcoin holders have not incentive to sell bitcoins for any fiat (at any price), Bitcoins should escape the reach of this attacker for good.