One of the reasons for the explosion in mobile telecoms in Africa is the ability to swap credit by SMS. It's allowed the operation of small economies in the backwaters of certain sub saharan areas.
Now one of the market leaders (I think, don't know this for sure) has allowed cross border SMS swaps. This means that for some people (and probably a sizeable slice of the population), SMS credit will become as important as the local currency.
Given a large proportion of these economies are fully interchangable with dollars at a street level, should mean that the local people rely on the local currency to a lesser extent.
Thinking about it, if your income is decided by what you can grow and trade, you don't need the local currency. In some ways you could probably get away with no currency at all.