Bitcoins could never replace fiat currency. Why? because governments wont allow it to, just like US government wouldn't allow Saddam Hussein to denominate Iraqi oil in Euros.
Oh well that's a really true fact , if a country want they can surely adopt for the use of bitcoins in their country but yeah if any of the country decides to adopt bitcoin for foreign trade it would be a indirect call for us to invade them. I actually feel that the main use of bitcoin is for foreign trade and not adoption of it as a fiat , but until us is there none of this will be ever possible.