Come to think of it, if the US government wanted to kill bitcoin today that'd be the way to do it. Ask the NSA to switch their cracking lab to mining for a few hours over a weekend, rack the difficulty up to say 20-30 million and bitcoin is history. The fact that it hasn't been done proves to me that the US gov't, at least, is utterly unconcerned with bitcoin.
I don't think the NSA has enough money for that or GPUs currently available. The difficulty can also only increase 4x per increment.
Since when was there a limit on the increase per difficulty adjustment? I thought it was proportional to the time allotted of 2 weeks, 6 blocks per hour...