Ever since Beta 2 of the 2007 Microsoft Office System came out, the amount of content around the new file formats has been growing rapidly. And I keep finding cool new resources that I didn't know existed.
Here's a great one: a 22-page article on MSDN by Erika Ehrli entitled "Walkthrough: Word 2007 XML Format." It provides an overview of the Open Packaging Convention, parts, relationships, content types, the custom XML data store, and an example of how to create a WordprocessingML document from the ground up. It's a good background article to read if you're going to start writing code to work with the Open XML file formats, or if you just want a better understanding of the details.
That article is just one of many useful XML development resources you'll find on the XML in Office Developer Portal.