they have lots of super-computers to make a lot of floating points operations that, if they want it today, they can make the 51% attack.
but really, what are the chances of that ever happening? 
If Bitcoins really become a dominant force and threaten any of the major currencies, is it not possible that they would launch a 51% attack and control or destroy the bitcoin system?
When bitcoins threaten any major currency or the government control of economy, then they will have an incentive to make the 51% attack. That's when the chances of it happening go up.
I am still interested in knowing if there is anything in the technology that can thwart this.