Fears and warnings about the consequences of a single entity obtaining majority network power have been known for some time, but have generally been dismissed as not conveying enough control to be worthwhile. For the first time, and for several extended periods, the GHash mining pool delivered 51% of the bitcoin network hashing power, despite promises that they would never cross the 50% threshold. Although GHash did not take advantage of its monopoly power during these times, it does seem that we have crossed a threshold. How do you trust the blockchain to an anonymous monopoly?
Ars is reporting on a statement by the CIO of GHash [arstechnica.com].
In the same article, they reiterate the two theoretical attacks: 1. Double spending bitcoin, and 2: Prevent new transactions.
This really highlighted to me the possibility that many governments probably "turn on" enough computing power to obtian 51%, and seriously damage any *coin anytime it wanted.
In other words: bitcoin and others are not, and will never be, a threat to governmental control of currency.