Microsoft continues to invest deeply in XML in Office. The new release of Microsoft Office "12" Word®, Microsoft Office "12" Excel®, and Microsoft Office "12" PowerPoint® includes a set of XML schemas that comply with the W3C specifications. The XML schemas allow you to understand an XML based definition of Word documents, Excel spreadsheets, and PowerPoint presentations as part of the new Open XML formats for Office "12."
If you want to understand more of the internals of Microsoft Office "12" Word®, Microsoft Office "12" Excel®, and Microsoft Office "12" PowerPoint®, download the Office "12" XML Schema Reference - PDC 2005 Preview and take a deep dive into the XML schemas.
Keep in mind that this is draft documentation and that some details of the schemas might change for the Beta release.
Have a peek and enjoy!