The clever mobile payment system that Starbucks has been testing in a few stores in Seattle, New York and Silicon Valley is going national.

Starbucks is announcing that it has expanded the "pay by phone" program to 6,800 of its stores, plus more than 1,000 outlets inside Target stores. It began testing the system at a few stores in September 2009.

To use the system, Starbucks cardholders load an application onto their iPhone or BlackBerry smartphones. The application displays a barcode that's scanned at the register to pay for drinks. Users can also manage Starbucks accounts and find nearby stores with the application.

With the launch, Starbucks claims to be operating the largest mobile payment program in the country, giving millions of cardholders a fast way to pay for their jolt and a muffin.

One in five Starbucks transactions is now made with the store cards, and mobile payments "will extend the way our customers experience and use their Starbucks Card," Brady Brewer, vice president of card and brand loyalty, said in a release. "With mobile payment, the Starbucks Card platform further elevates the customer experience by delivering convenience, rewarding loyalty and continuing to build an emotional connection with our customers."

Customers apparently like using Starbucks Cards. They loaded more than $1.5 billion onto the cards last year, up 21 percent over 2009.

They also like smartphones. Starbucks said more than a third of its U.S. customers use the devices, and nearly three fourths of the smartphone-toting Starbucks customers have either an iPhone or a BlackBerry.

A version of the Starbucks application for Android-based phones is being developed. There's no word yet on plans for a Windows Phone version.