builds a that plugs into the headphone jack of a smartphone. The company takes 2.75 percent of each sale. Apple recently began selling the card reader online, and in it's stores for $9.95. But users get a $10 Square credit when they sign up for an account and the apps are free. The company is adding about 100,000 local merchants a month, says Keith Rabois, Square's chief operating officer. In the first three months of this year, Square processed $66 million in payments, and it expects to triple that amount in the current quarter, he says.

But the new industry faces some hurdles.

Two months ago, VeriFone, which makes a competing credit card reader, wrote an open letter to consumers and the industry, warning users of a security hole in Square's hardware. The company denied the allegations, with many journalists pointing out the vulnerability of credit cards is a universal issue, and that Square's system is not uniquely insecure. Near Field Technology (NFC) for example, a similarly that allow users to make a purchase by only using their phone, is subject to similar security issues.

As with all emerging industries however, there are going to be problems. Square and VeriFone have to convince businesses--not to mention customers--that mobile credit card processors are safe.

Visa's investment and partnership will go a long way in assuaging those fears.