“One of the key goals of the collaboration effort is to build file format conversion technology that will provide greater interoperability between the OpenDocument and Open XML file formats. Novell and Microsoft are not trying to develop a file format that is optimized to work only with a particular version of Open Office, Ballmer said.
Nor will the collaboration team attempt to build file converters that can make files 100 percent compatible between the two file formats, he said. But it will achieve the level of interoperability that customers can work with, he said.”
See, there is never any doubt that MS will make a good and proper compatibility with ooxml and odf. Heck, their own products do not even do ooxml properly.