An acquisition by American Express in Septembergave it a piece of a business that analysts say other lenders should jump into: virtual currencies and virtual commerce.

American Express paid $30 million for Sometrics, a four-year-old company that helps video game makers establish virtual currencies and virtual currency commerce within their games.

Why pay $30 million for an enterprise that helps video gamers buy virtual clothes for their online avatars?

Measured in real-world U.S. dollars, virtual transactions total $2.1 billion a year in the U.S. and $7.3 billion globally, according to Sometrics' founder and chief executive, Ian Swanson. That is up 61% since 2008.

"It's growing fast. It's only picked up steam as more and more companies are getting into the virtual currency market," Swanson says. His El Segundo, Calif., companyworks with 450 video game companies serving 250 million game players in 180 countries. Amex is expected to build out virtual currency platforms in other industries, capitalizing on its merchant relationships.

Virtual currency is a broad category that can include everything from the money used in online games to digital currencies used to by real-world goods to Starbucks points.

Video gameplayers use actual money to buy virtual-world units of currency. The units are then set aside in an account to be used to buy items while playing the game — virtual armor, clothes, farm animals or real estate, for example. Facebook users buy Facebook Credits, priced at 10 cents a piece, to fund an account that is debited as the user buys from applications on Facebook. Microsoft sells Microsoft Points for users of its Xbox LIVE.

Bitcoin is digital currency that can be used for real-world peer-to-peer purchases, as well as for currency speculation, just like the exchange-rate traders. Starbucks points, airline miles and other loyalty programs could also fit the virtual currency definition.

The developing field of virtual currency has caught a lot of regulators and banks by surprise, says Alistair Newton, a Gartner banking analyst. But the banking and financial services companies, starting with the credit card companies,are waking up to the business threat and the market opportunity, he says.

The Amex acquisition is one example. Another is Visa Inc.'s February purchase of PlaySpan Inc., a virtual-goods payment platform, for $190 million.

Where do banks fit in? As banks look to develop strong and immediate relationships with their customers, they could present themselves as stores of value, where the customer could deposit airline miles and Facebook Credits in accounts alongside his savings account and checking account, Newton says.

A customer waiting in line at a Wal-Mart could decide, with the help of a mobile phone application and his bank-kept accounts, how best to use points or a debit card account to make a purchase.

Online gaming currencies make up a subcategory that "may still stay ring-fenced," but loyalty points and other forms of virtual currency that can be used to purchase tangible goodscould spill out into real-world banking, Newton says.

The potential profit margins in the exchange business with virtual currencies should interest any banker, Newton says, buthe knew of no banks in the public domain offering services related to virtual currencies.

In 18 months, depending on how the global banking/economic crisis plays out, the industry "may start to see the tip of the iceberg emerging" with banks offering virtual currency services, Newton says. "There's not going to be an overnight changing of the markets."

Concerns about regulatory and reputational risk may partly explain the holdup, says Zilvinas Bareisis, a senior analyst within Celent's banking group. Because alternative currencies operate outside of the regulatory framework, banks would run into significant potential risks in trying to administer related accounts or services, he says.

Another problem would be accounting for virtual currencies, Zilvinas says: What would be the generally accepted accounting principles for keeping virtual currency accounts on the books?

Some of the first regulations for virtual currencies are expected to involve anti-money-laundering rules and to come out of the U.S. Treasury Department. Microsoft recently petitioned the Reserve Bank in Australia to develop a system to allow for exchanges between virtual currencies.

Swanson says regulatory and compliance issues are one reason his company did not get into the business of helping game users cash out of virtual currencies. Sometrics deals only with the buying-in sideright now. "That was not an area that we wanted to move into as a startup," Swanson says.