After much speculation, Facebook is about to start closed beta testing its payment system for application developers.
This is big news for developers seeking to monetize their apps through transactions. Transaction-based applications that use virtual currencies or kick users out to Paypal could soon start accepting payments from users directly in their apps.
A Platform payment system could also be a significant revenue generator for Facebook. While Facebook is not making money directly from application use (developers keep 100% of ad sales), developers would be happy to pay a commission for the service.
Developers who are interested in participating in the beta test can go to the Developer Forums and email the Facebook Platform team. You will be required to sign an NDA to participate.