I saw this question asked internally today, so I thought I would share.  If you want to develop to use the Open XML document format you probably want to look at two things.  The Microsoft resources (eg http://msdn2.microsoft.com/en-gb/office/aa905545.aspx) and the ECMA resources Standard ECMA-376.  On the MSDN site you will find documentation and a MS SDK (less interest if you are developing using a functional language for your mainframe ).  The ECMA site has the following:

Standard ECMA-376
Office Open XML File Formats

(December 2006)

This Standard defines Office Open XML's vocabularies and document representation and packaging. It also specifies requirements for consumers and producers of Office Open XML.

An Office Open XML overview is available on the Ecma website.

Copy these file(s), free of charge:

File name	Size (Bytes)	Content
ECMA-376 Part 1	350 630	zipped DOCX file
ECMA-376 Part 1	1 441 795	zipped PDF file
ECMA-376 Part 2	341 625	zipped DOCX file with 2 Annexes
ECMA-376 Part 2	1 286 494	zipped PDF file with 2 Annexes
ECMA-376 Part 3	6 625 515	zipped DOCX file
ECMA-376 Part 3	5 930 003	zipped PDF file
ECMA-376 Part 4	14 477 799	zipped DOCX file with 4 Annexes
ECMA-376 Part 4	29 008 573	zipped PDF file with 4 Annexes
ECMA-376 Part 5	99 655	zipped DOCX file
ECMA-376 Part 5	687 279	zipped PDF file

Just thought I would share for those who know I am a big fan of easy access to the innards of document formats.

ttfn

David