UPDATE: Here's the OpenOffice.org's official press release on the topic.

By announcing that its new format would be standardized, open, and XML based, Microsoft has also announced to the world that it had lost the initiative in the Office wars.
Microsoft latest move is the evidence that OpenOffice.org, and the OpenDocument Format are leading the way when it comes to innovation in the Office technology and the vendor-independant data formats. So thank you, Microsoft.

Thank you Microsoft? Actually, not that much. Whereas the validation of the MS Office next formats by the ECMA and the ISO remains to be seen, I'd like to point out that Microsoft did not choose the existing, state-of-the art open file format, OpenDocument. By choosing not to adopt OpenDocument and claiming for another standards, Microsoft has opened a WAR OF STANDARDS. A war of standards is exactly what Microsoft wants in order to stop, or try to stop the momentum behind OpenDocument.

Some may also object that these upcoming MS Office file formats are actually open. But it may not be the case if you consider these points:
-Microsoft truly opened its format, but did not open its former versions of the MS Office file formats.
-the openness of the file format has to be narrowed: open, according to Microsoft, is not open as in "open source" and the licensing scheme, IPR statements from Redmond are a progress but fall short of any comparison even with Sun's CDDL or BSD. Beside, Microsoft patented the XML technology used in its formats. What good is that?
-Microsoft, despite its claims, doesn't invite others and the community to work and decide collectively on the format specifications. Microsoft owns it anyway, and can change it overnight.

As for myself, I suggest that we keep pushing OpenDocument, no matter what Microsoft says and does. Let's keep an eye on Redmond, though. As time goes by, it may even release Windows under GPL...