Are you kidding me?  Our network is roughly 4 million dollars worth of 5830s.  NSA & friends are NOT using graphics cards, they have had dedicated hardware for every major crypto even before the algorithms become popular.  Buildings full of hardware.

If the US government was interested in disrupting bitcoins as opposed to attending to other matters of national security you'd be looking at tens of millions in difficulty after a weekend, way more if they got serious.

The US government throws millions of $ at various pork projects non-stop.  If BTC were seen as a threat to national security or even domestic policy -- well, let's take a modest budget of 10 to 20 million dollars.  About the same price as an average supercomputer.  Look at the ASIC thread to see what kind of hash rates can reasonably be expected from a 2 million R&D investment, spend the remaining 8-18 mil on manufacturing 5 gigahash devices for 500 bucks a pop and there you go.  Conservatively 80-180 terahash on a budget even a municipality could swing.  In reality volume chip pricing mean you'd be looking at 500 terahash and a difficulty approaching 100 million for that 20 meg.

To put this in further perspective, the "Bridge to nowhere" project cost 400 million.

Still think the current 14 terahash looks invincible?