That's the first thing authorities (anyone who has extreme power and chooses to use it conflicting with what others want) would do, which leads to the wars I described, and they would be forced to change their minds by the many countries who have an interest in them not being dictator through the central bank system. This may not happen until more countries notice the change happening in the global economy, but they will eventually learn of it. Since authorities would change their mind after making it illegal, on average authorities don't really want it to be illegal. This is about slavery to money, so don't think it will be ignored when there is a way to end slavery. Its less about technology and more about many countries being the slaves of a few others, and about individual slaves and bank masters. Free the slaves or fight the biggest war in all of history.
I don't think you've stepped back quite far enough, Ben. You seem to be holding that the "Central Bank" is the great evil. Certainly central banks and their maniuplation of the the currency to the benefits of the banks is a problem, but it's not the central banks that start wars (on their own people or other nations), setup massive internal spying apparatus (alla the KGB, and now the American Dept. of Homeland Security), or back their country's fiat currencies. Those are government operations. While it's true that central banks would have a lot to lose from a peer to peer currency like bitcoin from going main stream, governments around the world have even more to fear. They lose their tax base, they lose their strangle hold on their population when they lose their ability to "follow the money", they lose their ability to socially engineer everything from tooth paste to tighty-whities.
There has yet to be a government created in human history that has not sought to increase its power over its people. Historically force was used. Today it's social manipulation. Get everybody begging to plunder everybody else, with the government in the middle, and you've got a government capable of almost anything becuase it's legitimacy really does come from its constituencies. And that's exactly what we have today in the U.S. and in Europe. Socialized democracies who prey on their consitituencies fear to feed their endless thirst for power and control. Take from this group and give it to that group. Take from that other group and give it to those people.
And the inexorable end to that ponzi scheme is close at hand: The growing debt crisis in both Europe and the USA.