According to the latest Nielsen research, commerce over mobile devices such as cell phones is accelerating in the United States. Nielsen Mobile reported that already 9 million U.S. mobile subscribers say they have used their mobile phone to pay for goods or services, and half of all data users say they expect to participate in mobile commerce in the future.

Nielsen’s research found that men are more likely than women to use their phone for commerce: 4.5% (4.9 million) of men and 3% (4.3 million) of women say they have made a purchase using their phone. Adults aged 25-34 are the most likely to have made a purchase using their phone: 5.4% (3 million) of adults in that age bracket have made a mobile purchase, compared to 3.6% of all mobile subscribers.

Users frequently visit mobile Websites to make purchases from their handsets; of the 40 million active U.S. data users in April, 5 million accessed mobile shopping and auction sites — up 73% from April 2007, when just 2.9 million mobile users did so. Auction site eBay is the most popular shopping destination on the mobile Web, with 3.4 million unique visitors in April.

Users are also beginning to purchase items using text messaging, the research firm found. Some services allow consumers to send text messages to a phone number or mobile short code in order to be charged for goods or services directly on their mobile phone bills. Already, 6.5 million U.S. mobile consumers say they've used text messaging to purchase an item.

“For many of the millions of consumers who are already shopping online or over landline phones, mobile commerce is an obvious and useful extension of that opportunity,” Nic Covey, director of Insights at Nielsen Mobile, said in a statement. “As more mobile commerce services become available and consumers develop a greater trust for phone-based transactions, we expect commerce to be an increasingly important part of the mobile experience next year and beyond.”

The study also found that security is the number one concern among those mobile data users not yet participating in m-commerce: 41% of data users who do not participate in mobile commerce say security is their biggest concern, 23% say they worry about being charged for the airtime and 21% say they don't trust that the transaction will be completed.

Data about mobile commerce in the U.S. comes from Nielsen Mobile's monthly Mobile Insights survey of more than 30,000 U.S. wireless subscribers, with similar data available internationally.