Get the InDesign CS4 SDK
The new InDesign CS4 SDK is available here. If you click on the Documentation tab, you'll find the links to the IDML documentation at the bottom of the page.
XML is a beautiful thing...primarily for geeky people who are passionate about topics like workflow automation and industrial strength publishing solutions. Being able to describe document geometry and document content in a non-binary file format that can be edited and manipulated by databases, scripts, and other processes enables a vast universe of vertical market solutions that can save publishers a lot of time and money, as well as offer opportunities for new types of publishing products.