Don't you need computer/internet access to make use of Bitcoin? I'd say that rules out a lot of people, especially in third-world countries.
In 2009:
Quote
over 65 percent of the African population had access to mobile phone service, with. 93 percent covered in North Africa and 60 percent in sub-Saharan Africa
SourceAll mobile phones will be internet-enabled in 10 years, and I think it is pretty safe to assume that most people on the planet will have some access. It won't surprise me if bitcoin first goes mainstream in an up-and-coming third-world country or region.