As much computing power as the entire history of computing power spent on the block chain. They don't just need to exceed the amount of computing power working on bitcoin today, but all of it that has been put into bitcoin since it was started, put together.

Even if that did happen, I think bitcoin could recover. All you'd need to do is get everyone to agree on a point in time where the blockchain was untarnished and to reset to then. That would be easier than it sounds, given the small number of existing clients. It would be damaging, but only in the short term.