The frontend API is pratically stable, but new additions will come. A new image loading system was designed and implemented (0.7.1). A wrapper to a GUI system is being designed, so modules can interact with the user.
The internal API is being done solidly, to avoid future problems. I'm taking special care to make sure that it's a good system, and will support the rest well. There's a real bunch of fprintf's to the inevitable debugging. ;-)
The module API is not stable. It's being developed. The general idea, however, is here.