According to PayPal Mobile Director of Product Management, Fabio Sisinni, "We are doing more than $6 million per day in mobile transactions and we’re on track to exceed $2 billion this year.

"We will top $7.5 billion in 2013 based on the growth we’re seeing. A mobile wallet in the cloud is not so distant based on our numbers."

That's serious growth. In fact, PayPal's own figures show it to be an increase of more than 250% from 2010. And, based on the current rate of growth, PayPal estimates that number to reach $7.5 billion in 2013.

So where are all these mobile payments taking place? Merchants using Express Checkout include Starbucks, Footlocker and Overstock.com, and of course eBay is using PayPal mobile. Massive growth potential is also seen in the physical retail world. For example, visit a store after receiving a coupon on a mobile device then redeem said coupon at the store -- technically, a mobile transaction.

Another driving factor in mobile payment growth might be the sudden increase in mobile search, as detailed in this Performics study.