I've got family in Africa. You'd be pretty goddamn amazed where great coverage appears. I agree that it's kinda shitty in a lot of places but that's gonna change.
I also travel into these areas too for contract work, and I am amazed at how strong the cellular networks are in these areas. Smartphones are becoming cheaper and more affordable and these people just love their phones and spend a large portion of their income on communication.
A company like 21 Inc will also come up with a solution for the unbanked in collaboration with bigger companies like Google and the Bill Gates foundation. ^smile^