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Smartphone owners in the U.S. bought $500 million worth of goods and services using mobile wallet apps in 2012, according to a new report by Berg Insight. That’s just a drop in the ocean compared to the trillions of dollars in credit and debit card transactions every year, but what’s most surprising is where those mobile payments took place.

According to Berg Insight, the vast majority of in-store mobile wallet transactions occurred in a Starbucks using the coffer giant’s (s sbux) own smartphone card app. Google Wallet(s goog), Square Wallet, LevelUp, Dwolla may be getting lots of media attention, but a single app that lets you buy coffee and pastries at a single business’s retail outlets is ruling the roost.

This is actually less surprising than it might sound. The problems with mobile wallet services today are that you don’t know what merchants will support them and actually closing a transaction with a supporting retailer is often more difficult than it sounds. Also, there’s nothing broken about plastic. In most cases, it’s far easier to whip a credit or debit card out of your pocket than present a QR code or NFC-powered smartphone to a baffled store employee.

On the other hand, Starbucks’ mobile wallet integration is as top down as you can get. You’re pretty much assured you can walk into any Starbucks in the U.S., present your phone and walk out with a mocha latte. Starbucks CEO Howard Schultz recently said that the company is processing 3 million mobile transactions each week, though some of them are on Square Wallet.

Berg, however, views Starbucks more as a trailblazer than the pre-ordained king of mobile payments. As they buy more croissants with the Starbucks app, consumers will want to buy other goods, too. Berg projects the mobile wallet market will grow into a $35 billion market in 2017 (still tiny compared to overall credit/debit card use), but it believes that other digital billfolds — such as Google’s, the mobile carriers’ Isis, and the major retailers’ own MCX – will drive the majority of in-store mobile wallet transactions.

In Europe, the mobile wallet is buried even further in laundry pile. Berg calculated that 29 European countries generated only €100 million (U.S. $130 million) in wallet transactions in 2012. Berg, however, expects Europe to embrace mobile payments more readily than Americans, projecting the wallet market to grow to €45 billion in 2017.

Feature image courtesy of Lucid Design Group and Yehohanan92 via flickr