Namita Gupta posted more details about the change earlier today:
This is a relatively significant change as FBML has long been a core component of Facebook’s Platform. More recently however, many developers that I’ve spoken to have been working primarily with iFrame applications anyways. For those who prefer FBML, you’ll need to begin making the shift ASAP. This is part of a broader push toward the new Platform APIs and away from the old structure. As this shift takes place Facebook will also eventually pull the developer wiki.
All of the upcoming changes are reflected in a new developer roadmap (found here).