During the first half of 2007, trainers from Microsoft, Sonata, InfoSupport, Allette, Predeek Consulting and others delivered Open XML developer workshops in over 30 countries. These workshops were all structured around a set of inter-related presentations and hands-on labs that cover typical Open XML development scenarios.
Those presentations and hands-on labs (as well as all supporting content, such as demo files) are now available for free download from OpenXMLDeveloper.org at the links below.
ADDED 8/14/07: there is now a book available as well that covers many of these topics. You can download the book here.
For help learning this content, please post your technical questions on the Forums section of OpenXMLDeveloper.org. Many of the persons who created this content are active participants in the forums and can answer your questions there.