An excellent Economist special report – see the article links on the leader page. An excerpt:
The reason why mobile phones are so valuable to people in the poor world is that they are providing access to telecommunications for the very first time, rather than just being portable adjuncts to existing fixed-line phones, as in the rich world. “For you it was incremental—here it’s revolutionary,” says Isaac Nsereko of MTN, Africa’s biggest operator. According to a recent study, adding an extra ten mobile phones per 100 people in a typical developing country boosts growth in GDP per person by 0.8 percentage points.
From another article in the special report, “Finishing the Job”
In the grand scheme of telecoms history, mobile phones have made a bigger difference to the lives of more people, more quickly, than any previous technology. They have spread the fastest and proved the easiest and cheapest to adopt. It is now clear that the long process of connecting everyone on Earth to a global telecommunications network, which began with the invention of the telegraph in 1791, is on the verge of being completed. Mobile phones will have done more than anything else to advance the democratisation of telecoms, and all the advantages that come with it.