You can read Interoperability, Choice and Open XML on Microsoft's Web site. It is entertaining, if not spectacularly even-handed -- it claims, for example, that IBM is trying to "force ODF on users through public procurement mandates," while it's not exactly forthcoming about details like how much it might have spent on lobbying in its struggle to keep Microsoft Office on the official purchase schedules in places like the state of Massachusetts.
ODF is an open standard -- is was created under the watchful eye of OASIS and published by both ISO and IEC. Open XML is an open standard in the making. Microsoft has do something to keep from being driven out of the office-suite business. Governments are leading the way in demanding open document formats -- especially in Europe, where Microsoft's obsessive proprietism, if that's a word, has gotten it in trouble with courts over and over.
It could very easily solve its problem by embracing ODF and incorporating support for it into its products -- except that would involve agreeing to licensing terms Microsoft simply will not accept (can you say "open source"?)
So the software giant's only alternative is to create a standard of its own. A good bit of the letter is devoted to creating the impression that Open XML is just like ODF -- that is, open -- but better. ODF is "closely tied to OpenOffice and related products, and reflects the functionality in those products," says the letter, while "Open XML, on the other hand, reflects the rich set of capabilities in Office 2007." And is closely tied to just one product, Microsoft Office, which Microsoft seems to have forgotten.
It's not just IBM that's objecting to Open XML, and the objections are not sibling rivalry. You can see a summary of the objections on GrokDoc. (Be advised that GrokDoc is an offshoot of GrokLaw, which is currently having propaganda-related problems of its own.)
Many of the objections seem to be deeply technical responses to things in the Open XML spec like " 'Application-defined' binary blobs for Microsoft Ink&tm; data." Steven G. Johnson, who identifies himself as an assistant professor of applied mathematics at MIT, notes, "This is obviously a thinly-veiled way to include the proprietary Microsoft Ink tablet-PC annotation content in [the standards document], without documenting the format used."
There is a "Through the Looking-Glass" quality to all of this: Microsoft has attacked ODF as being somehow less open, because it is supported by IBM, than Open XML, which apparently at this point still harbors Microsoft technology that is extremely likely to ever be released to an open software environment on terms acceptable to the open-source community.
It's a truism in the software business that "the good thing about standards is that there are so many of them," but I'm inclined to hope that Sun's plan to write ODF support for Microsoft Office succeeds. ("We've done what Microsoft could and should have done in the first place instead of FUD-ing and fighting," blogged Sun's chief open-source officer). Do we really need another open document-format standard? I'm glad Microsoft is feeling generous, but experience make me inclined to look this gift horse in the mouth.