IMHO, the key to Microsoft's OOXML strategy can be seen in the recently released MSOffice SDK. The SDK provides a component for the fluid conversion of OOXML to something called fixed/flow. The fixed part of this interesting conjunction is also known as XPS, which is designed as a proprietary alternative to PDF. The flow part is a fascinating and highly proprietary replacement for (X)HTML - CSS.
Reading further through the MSOffice SDK, one can't help but be amazed at the lack of W3C technologies; especially (X)HTML, CSS, XForms and SVG. What we have instead is an entangling cascade of stuff like OOXML, fixed/flow, silverlight, XAML, and WPF. And then there is that recent promise of other high volume API's probably delivered through future Exchange, SharePoint, and MS SQL Server SDK's.
So, at the end of the day, what are we looking at here? IMHO, Microsoft has figured out that the smart thing to do is leverage and extend their existing desktop monopoly into the next generation of cloud computing where the Internet platform rules.
To pull this off, they have a number of problems to overcome; not the least of which is that they need to catch a break on anti trust, and, get OOXML through ISO. And oh yeah, there's that little problem that Windows can't do cloud computing.