Mashable has learned about the launch and has details about the app itself. The launch was decided to be part of the Apple event, because Facebook didn't want to take away from its own announcements at the recent f8 conference. The app looks like an extremely rich experience with a nice paneled layout, and access to most of the major features you'd expect. Although in the screenshots from Mashable, there doesn't seem to be integration for Facebook's new Timeline feature, and while there is built in chat, there doesn't seem to be any video chat.
Facebook and Apple haven't had the best relationship recently, so it may be a step in the right direction for the two that this announcement will happen at the Apple iPhone event. There are also rumors that Facebook will also unveil its "Project Spartan" at the same event. Spartan is the Facebook project to use HTML5 to allow developers to bring their Facebook apps to mobile devices. It seems we'll get all that information next week.