[Estragon]

The hope I've heard from cryptocurrency enthusiasts is that they stay under the radar until they're too big for any traditional organization to overpower the network.

It would certainly still be feasible for the US government to overpower the network, but it would not be cheap. The Minerscube 15 is due to ship next month, and will get you about 1.666 GH/s/$ [0]. The current speed of the bitcoin network is about 160,000 TH/s [1]. Duplicating this would cost about $96M, assuming you could get that many Minerscube 15 chips at that price, and assuming the infrastructure costs are trivial. So I would say it's still doable, but far from trivial.

  [0] https://en.bitcoin.it/wiki/Mining_hardware_comparison#ASIC
  [1] http://bitcoin.sipa.be/