Starbucks tested a mobile phone payment option last year in their stores that are located inside Target. It seems this was a succes, as you can now pay for your coffee in thousands more of their stores using you BlackBerry, iPod Touch and iPhone.
The way it works is by utilization of a bar code on the person's phone that is linked to their Starbucks card. The customer holds it up to be scanned by the Starbucks employee. The Starbucks card is funded by the customer's credit card. If the card runs out it can be refilled straight from the phone.
Starbucks is saying on their website that over 7,500 stores in the United States will be able to accept this form of payment, including the test stores that are in Target. They are working on allowing Android phones for payment with the same technology, but there is no word on when this will be ready.
There is also a company called Near Field Communication that is working on using short-radio communications for payments with mobile phones. This is already in use in Japan and other countries around the world, but widespread use in the U.S. is a while away.