Welcome to Wal-Mart. Would you like to open a bank account?

Nightmares of such a greeting have kept many community bankers up at night, and their lobby has fought hard to see that it doesn't happen. Yet the Bentonville, Ark., behemoth has found a back door into certain banking services. While the company that helped open it, Green Dot, has profited handsomely, investors may have missed their opportunity.

Green Dot is the largest provider of prepaid debit cards, which are sold as an alternative to checking accounts. Consumers can cash checks and have funds loaded on the card, set up direct deposit, pay bills and more. With many low-income consumers underserved by traditional banks and increasingly unable to get credit cards, prepaid debit is exploding. Industry forecaster Mercator Advisory Group expects funds "loaded" onto such cards to grow more than 60% annually through 2013.

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