Many still use Office 2000, not the ideal software to add a .docx compatibility patch to. Open Office however has first class legacy file support.
Circumstantially it looks very much like MS's strategists are relying on 'just one more generation' of Office users before revenue streams from the on-line Web 2.0 world crank up. The strategy may not work for another reason though, one that MS is acutely aware judging by the resources it is committing.
Uptake even of free software has its own problems.
The Free Open Source software world has always struggled with the lack of cost of its products! Marketing 'free-stuff' as enterprise quality equivalents to 'very expensive-stuff' is not always easy as those of us in this industry know very well. It's counter intuitive and a lot of breath gets wasted explaining how FOSS even got to exist at all let alone how it became so good.
Open Office itself gets better each version, but soon I guess it too will be as glossy and as over featured as MS Office. Then how do you choose between two products other than by familiarity and personal preference? Why also would you stay with one product, unless you were locked-in by some odd format?
A strong feature of high quality Open Source Software has been adherence to open standards and the endorsement of really major companies supporting such standards. Factors like open standards have enormously helped the deployment of OSS solutions into industry. It follows that software with idiosyncratic non-standard file formats can't even be given away...
... now I understand. That's what all the fuss is about: ODF versus OOXML! No standard means no product differentiator which means dwindling market share even when you give it away. Exciting stuff.