Old thread is below for archiving's sake. This post content is now out of date - the above link is now the latest version of the SDK documentation.
Also, user Kyatric has kindly compiled a HTML version of the SDK documentation viewable online.
Last update: 28th November 2011
This is currently some very lo-fi text files, but the content is there. There are also templates to help get you started with a plugin or behavior. In future, it'll all be hosted on the site, linked up, and so on.
Let me know if you have any problems, find anything confusing, or get stuck - we're more than happy to help!