I just found the OpenXMLDeveloper.org site. Cool! I found the site after searching live.com for more information on open xml formats. There's a great article that provides source code for a simple tool to generate text in XML and store it in .docx (Word 12).
Just yesterday I spoke with a university about the changes in document formats including Office files and XPS. This site provides real evidence of all the work Microsoft is doing with these file types.