Chris Davies writes:
"As the mobile app provision increases, Square and Starbucks will make buying a cup of coffee even more straightforward. Passbook [on your iPhone] will automatically flag up the Starbucks Card on the iOS device’s lock screen when they enter a store, and they’ll be able to pay by “swiping” their phone."
I've been using Passbook for my Starbucks payments and like it. It launches instantly, while the Starbucks iPhone app tends to be a bit sluggish.