A first version of documentation for the scripting API - the API you write games/activities in, also known as "the API" ;) - has been uploaded to docs/. You can see it here. It has a 'hello world' example, a tutorial, and a few big examples of actual working code (a plugin, an activity, etc.) with detailed explanations. This should be enough for people to get started with scripting.
We don't expect to have complete documentation in time for the launch, but with this document and the work-in-progress wiki, we should have enough for now. (The source code itself also has comments, particularly in the important places, and the example games and activities are helpful as well.)
EDIT: We moved to github meanwhile, the latest docs are here: http://github.com/kripken/intensityengine/blob/master/docs/scripting_docs.html