A couple of days ago the Open XML Format SDK was posted on Microsofts download site:

http://www.microsoft.com/downloads/details.aspx?FamilyID=ad0b72fb-4a1d-4c52-bdb5-7dd7e816d046&displaylang=en

With this SDK you get a set of strongly typed part classes for use with Open XML documents

Here's more info:

http://blogs.msdn.com/erikaehrli/archive/2008/06/10/announcing-the-open-xml-format-sdk-1-0.aspx