Although logically an extension of the Google Checkout service, fuel for the gPhone rumours can be found in a recent Google patent.
In February this year Google patented the "GPay" system which lets you order and pay for products and services via SMS. The patent summary reads:
"A computer-implemented method of effectuating an electronic on-line payment includes receiving at a computer server system a text message from a payor containing a payment request representing a payment amount sent by a payor device operating independently of the computer server system, determining a payment amount associated with the text message and debiting a payor account for an amount corresponding to the amount of the payment request, and crediting an account of a payee that is independent of the computer server system."
Very simply put, using SMS a customer can send a text message, example: "gbuy hardees401 6.25 7 yugo" to a shop to pay for low value products and services.
Their account is debited and the shop gets another message to say the payment has gone through.
One application mentioned is to use this to pre-order and pay for low value items in advance so they are ready when you arrive at the shop.
So, as an further example, you could text and order and pay for your coffee before you get to the shop so you can just grab it and go.
Much more detail in the patent document, link below...