2/3 of smartphone owners in the US used their phones for shopping-related activities in September 2011

"The results showed that 38 percent of smartphone owners have used their phone to make a purchase at least once in the course of their device ownership. The most popular products purchased on smartphones during the month of September included digital goods, clothing/accessories, tickets and daily deals.
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“Fueled by smartphone adoption, mobile is becoming a central part in the shopping funnel for many consumers,” said Mark Donovan, comScore senior vice president for mobile. “In September we saw two-thirds of all smartphone owners perform shopping activities on their phones, including comparing products and prices, searching for coupons, taking product pictures or locating a retail store. Considering there are currently 90 million smartphone owners in the U.S., retailers without a well-developed mobile strategy are not only missing a tremendous opportunity with these customers but also risk becoming obsolete in the minds of these digital omnivores.”"

Source:  Data from comScore reported in a press release, 5th December 2011