Nine million U.S. mobile-phone subscribers say they used their mobile devices to pay for goods or services in the first quarter ended March 31, according to a report by the Nielsen Co. Nielsen surveys 30,000 wireless subscribers monthly. Some 6.5 million consumers used text messaging to make purchases as of the end of April, according to the report. About 5 million used mobile shopping and auction sites in April, up 72.4% from 2.9 million in April 2007, the report says. "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," Nic Covey, director of insights at Nielsen Mobile, told attendees Thursday at the Internet Retailer Conference and Exhibition in Chicago. Security remains the primary concern among mobile-phone users who do not participate in mobile commerce, the study says. "U.S. consumers need proof that mobile transactions will be a safe, affordable and efficient complement to other modes of shopping," Covey says.