$25 billion worth of purchases were made with phones and tablets in the US in 2012

"In e-commerce, the mobile revolution is here.
In 2012, people spent $25 billion on purchases made from phones and tablets, an increase of 81 percent from the year before, according to eMarketer, which compiles data from 120 sources that track commerce.
That is still a minority of total e-commerce sales. Mobile accounted for just 11 percent of e-commerce and is expected to reach 15 percent this year. But eMarketer predicts that by 2016, mobile will be $87 billion, or a quarter of all e-commerce."

Source:  New York Times, 9th January 2013

Note - I'm assuming this doesn't include app purchases