It all boils down to the amount of people who adopt it as a stanard as measurement of wealth. if around 10% of the country were to use it as a form of payment (requesting it for wages, using it in shops and so forth) the likelyhood of it taking of would be signifcantly higher, below this it would never happen.
[EDIT - Found it] http://pre.aps.org/abstract/PRE/v84/i1/e011130
The other issue is government buying into the idea of a currency they could not control, although this would be relevantly null if they were to hold a substantial amount of any given currency, there is little reason they couldnt "mint" their own chain and distribute the funds.
Both ideas are entirely possible with widespread adoption but until the magic 10% figure is hit it will never happen.