Platform Update developers blog, the brief entry names Gamehouse's Collapse! Blast as the project's initial testbed, adding that interested developers should get in touch.
Rather than using Paypal or their credit cards, users now sign into Collapse! Blast via Facebook, at which point their existing Facebook Credits can be spent on virtual goods. Should the expansion progress beyond these initial testing phases, Facebook Credits may provide appsperience developers with a solution that's easier and cheaper to implement than a proprietary one. Facebook's massive existing install base doesn't hurt things either, and since adoption is the key to standardization, maybe this is the first step towards a Star Trek-esque credits-based utopian future society.