At the Mobile World Congress in Barcelona, all the talk was about mobile payments and the demise of out-of-date cash registers.

Anyone who has purchased anything recently in an Apple Store will testify to the superior customer experience provided by the roaming sales assistants with their handheld credit card readers. The transactions are seamless and in an instant an email appears in your inbox with a confirmation invoice. It saves queueing, time and money changing hands.

Now, the mass adoption of smartphones and tablets has set the stage for a new move - away from fixed-point, card-based transactions and toward those completed on mobile. The dream of the 'digital wallet' is finally coming true in a very particular mobile-led fashion.

In the U.S. mobile payments in-store nearly quadrupled last year: eMarketer has estimated in-store mobile payments as adding up to $640 million in transaction volume, up from $170 million in 2011. However, this figure does not include swipes on mobile credit card readers like Square and PayPal Here.

Starbucks is switching its credit and debit card processing to Square and as of January 2013 accepted the Square Wallet app at 7,000 locations. The Starbucks alliance with Square is evidence of how the card-reader approach has already worked its way deep into the U.S. consumer economy.

How long will it be before the Starbucks model is adopted by forward-thinking retailers over here?