28% of American mobile phone users and 48% of smartphone users used mobile banking in 2012

"A new study by the Federal Reserve Board revealed yet more strides being made in the area of mobile banking.
It found 28 per cent of all mobile phone users and 48 per cent of smartphone users had used mobile banking in the 12 months to Nov 2012. This is an increase from 21 per cent and 42 per cent respectively year on year.
The use of mobile financial services was more marked among the 10 per cent of the population that is underbanked (people with bank accounts but who use cheque cashers, payday lenders, or payroll cards).
90 per cent of these consumers have mobile phones, and 49 per cent had used mobile banking in the 12 months preceding November 2012, up from 29 per cent in December 2011.
The FRB reckons the same uptake might come next to the unbanked (those without a bank account), as 59 per cent of this group has a mobile phone, half of which are smartphones."

Source:  Mobile Money Revolution, 4th April 2013