Thanks to a comment from @Sorin, the official documentation is available here:
http://www.jetbrains.org/intellij/sdk/docs/index.html
Here are some docs:
Lots of links here:
http://confluence.jetbrains.net/display/IDEADEV/PluginDevelopment
The SDK:
http://confluence.jetbrains.net/display/IDEADEV/Getting+Started+with+Plugin+Development
Also, from the above link, you can check out the source code to the community edition, which should effectively act as API docs:
http://www.jetbrains.org/pages/viewpage.action?pageId=983225