Davidson said Starbucks has been the mobile payment innovator in the U.S. and has found that Americans will pay with their phones. "We're it, we're the test case, and we're seeing tremendous value," he said.

Moving to the Android is important because "Android is huge," he said.

Originally, Starbucks surveyed customers and found they wanted to be able to use an iPhone or BlackBerry for mobile payment; now they're clamoring for that capability for Android. Noting that the smartphone market changes almost monthly, he said, "Android is a huge growth opportunity for all mobile, so we're happy to be part of that now."

Along those lines, Davidson said Starbucks thought at first that "only trend folks with iPhones" would use the mobile app to buy drinks and food. "But we see now it's kids, old folks -- all over the map," he added. "We're surprised to see how broad the usage has been."

Davidson has visited many stores and watched as soccer moms and office workers paid by phone. "They don't have to dig around in a purse," he said.