Like other mobile-payment companies, Obopay is mindful that the world has four million mobile-phone users--and most of them are poor, use prepaid plans, and lack bank accounts. "We have a model that serves the one billion with bank accounts, and a model that serves the three billion without, and we give the same level of service to both and allow them to participate in the same payment network," Realini says.

India is an epicenter of this trend. More than 450 million Indians live below the poverty line, but more than 300 million Indians have cell phones. Some estimates suggest that India will have nearly 750 million cell-phone owners--the bulk of the newcomers from rural areas where banks are sparse--by late 2012. In terms of cross-border remittances, the United States is the leading source and India the leading recipient of such funds.

Another mobile service, mChek, based in Bangalore, says it now has one million users, mostly in India, who use the service to do things such as pay their cell-phone bills. "Nokia's investment is a validation of the coming of age of mobile payments and the expectation that this is now likely to become mainstream in the near term," says mChek's CEO, Sanjay Swamy.

Both Obopay and mChek are also expanding into so-called microloans, in which users are able to open bank accounts with tiny amounts and then execute payments to suppliers or customers with simple cell-phone clicks. Though a Nokia spokesperson declined to comment, in a written statement last week, Teppo Paavola, Nokia's head of corporate business development, said, "This investment reflects our belief in the global potential for mobile payments."