Fiserv, mcom and Syniverse published a report entitled: Mobile Channel Addresses Offline Consumers’ Needs for Frequent Transactions While Reducing Bank Channel Costs. 



In it they say that transaction security fears are at the top of the list when it comes to the most common barriers to using mobile banking.  Here's a snippet:



Mobile Banking ROI: Lowering the Cost to Serve



While mobile banking introduces some new operational costs, it is by far the lowest-cost banking channel in place today at an estimated $0.08 per transaction (figure 9).



Compare that to other banking channels such as call center ($3.75 per transaction), IVR ($1.25 per transaction) or ATM ($0.85 per transaction) and you can see how moving some of these transactions to the mobile channel could result in significant cost savings.



But consumers remain wary of mobile banking because of concerns about transaction security (figure 8). Security fears topped the list of reasons that would prevent respondents from using mobile banking with 53 percent.