The Open XML 1.0 SDK has been made available by Microsoft on its website
The version of Open XML supported by the SDK is not the ISO-ratified standard, but is instead the version of Open XML used by MS Office 2007. The newer version of Open XML will only be supported in the next version of MS Office due to the work involved in dealing with backward compatibility issues. However, the service pack for MS Office 2007, due in 2009H1, will add support for the ODF (Open Document Format used by Open Office), enabling users to use documents created with the open-source OpenOffice or StarOffice.
More details on the Open XML SDK 1.0 are available here: