Over the last few months, Doug, Wouter, Brian and lots of other people delivered a series of OpenXML workshops around the globe. Those who were there, and those who weren't, can now download all content of these workshops from OpenXMLdeveloper.org.
There are some interesting slideshows available for those of you who never heared about OpenXML before, as well as some nice code samples to get you started developing in C# or Java.
As a side note: I also spotted a good article on OpenXML, explaining the Open Packaging Conventions with a self-created file format (based on OpenXML).