After years of over-promise, mobile banking has hit a tipping point. With streams of customers in developed and developing countries alike growing swiftly, new business opportunities are pitting banks against tech companies, retail giants and others for control of the electronic wallet.

The Aite Group projects that, with rising smartphone use driving more consumers to mobile banking in the U.S., the number of people who will tap their bank account with a mobile device will rise from 33 million in 2012 to 96 million by 2016 – a 30% annual growth rate. The number of mobile bank customers will double over the next two years alone.

Mobile money is “not the next big thing — it is already a big thing,” says Tracey Weber, Citigroup’s managing director for consumer Internet and mobile banking in North America.

In developed countries, where smart phones increasingly prevail, banking and related services can go beyond simple transactions to include managing portfolios, retirement accounts and the like. What’s more, in the United States alone there are some 75 million people who either have no bank account or rely on nonbank services (such as pawn shops and payday loans) to get by. Many have some kind of mobile phone and so are potential mobile banking customers.

Yet, some of the most profitable gains for mobile banking will come in the developing world. In some countries, notably in Kenya, basic mobile phones have leapfrogged the substandard physical branch-banking system.  “We’ve gotten to the point where, in some countries, more people use phones for banking than use banks,” says Mauro Guillen, a management professor at Wharton. Kenya is one of them. More than half of the country’s 22 million adults use phone apps to do banking — twice as many as have bank accounts.

According to the World Bank, worldwide there are 1.8 billion people who have a mobile phone but no bank account. Millions of these people will be linked for the first time to the financial system by mobile-money applications. Notes Steven Lewis, lead analyst for Ernst & Young’s global banking and capital markets team, many millions of people in the emerging world are expected to join the middle class in the next few decades. The company — bank or telecom, or partnership — that brings them into mobile banking today stands to gain a long-term payoff in the future. “I would bet money that many will be upgrading to a smartphone before they get a bank account.”

As with any revolution, the old order is giving way to a new one, and banks are just one group of players on the mobile-money battlefield. Telecommunication companies, Internet and technology firms, retailers, and others are also in this fight. Factor in the marketing value of the data that can be gleaned from mobile transactions, and it’s clear why the new players could compete and win in this disruptive new digital space.

To delve into the future of mobile banking in more depth, Knowledge at Wharton this week published a free e-book (enhanced with videos), titled Mobile Banking – Financial Services Meet the Electronic Wallet. The book, sponsored by Ernst & Young, is available through Amazon Kindle, iBookstore, Nook and the Samsung Hub.