Edit: @Ein: If something being used as currency were to be cordoned off, the market would merely provide an alternative. It's like arguing that "Where would we buy cheap chinese goods without China?" or "Without Walmart?"
The first question that comes to mind is: why hasn't that happened for us?
Why hasn't what happened? Alternative currencies? Might have something to do with the fact that the government considers it economic terrorism.
But that's exactly my point; if some form of institution comes about for the purpose of cordoning off and regulating a currency's source material, it won't allow alternative currencies. You claim that the regulation of currency will simply result in new currencies being created; but at the same time, the organization regulating the currency will prevent this.
I'm not saying the market wouldn't provide a "better" alternative than government. I'm saying that the very institution of "currency" itself (i.e. a form of representative value that is abstracted from its source material) is opposed to an anarcho-capitalist, free market system. Any currency requires that its source be regulated, otherwise the reason for the currency becomes obsolete. If a free market, uninhibited by government regulation, decides on its own valuable material on which to base its currency, then an institution becomes necessary to protect that valuable material.
In order for a currency system to work, some form of central institution is necessary to maintain its value. If a free market decides on some valuable source that is "useful" (since, as we've established above, "useful" is mostly likely to secure the most value), it must deny this useful material from those who can't afford it. Ethically, this concerns me.
The only form of exchange I can practically imagine in an anarcho-capitalist society is the barter system.
I disagree with your assertions. First, you are assuming a "system". Of course a "currency system" needs centralization. Centralization is a problem the market would work towards and against simultaneously.
A protected monopoly on currency is as bad as a protected monopoly on anything else. Government is a de facto protected monopoly on many of it's functions.
The market would provide competing currencies, possibly until one proved most popular and gained a monopoly. At some point, it is most likely that this would lead it's producers to manipulate/abuse it, thus rendering it vulnerable to competition.