My point was that if "they" mount an attack, I think they're unlikely to do something subtle like try to mess with the block chain.
If "they" want to mess with the network, it would be far easier to do something like what China's doing with its Great Wall-- forcing connections they don't like to be dropped. Or what botnets do now: mount a denial-of-service attack by flooding the network.
Much more likely: "they" will catch you for tax evasion when you convert your bitcoins to your local fiat currency and buy that fancy sports car you've always wanted.