I posted on this earlier in the year, but I thought it was worth repeating, as I've seen a number of threads lately that question why Microsoft originally created Open XML rather than going with ODF. As you'll see from the timeline below, both formats were actually developed in parallel, and had very different design goals.
2005 – busy year
2006 – very busy year :-)
So, as you can see these formats were both developed over a long period of time. There were a number of lessons learned on each side, and of course the final results from each side are quite different given the differences in design goals. Folks on the ODF side wouldn't disagree with this. Gary Edwards, who played a big role in the ODF 1.0 standard and was one of only two people who participated in more than 75% of the meetings leading up to the completion of ODF 1.0 explains how interoperability with the legacy base of Office documents was actually blocked during ODF's development (read his comments under the post "Is It Game Over? – ODF Advocate Andy UpDegrove is Worried. Very Worried"):
Everyone on that first TC group supported full interoperability with Microsoft applications and documents, except for one company - Sun.
There are three areas of "interoperability" that Sun opposed then, and continues to oppose today. The only difference being that after their 2004 deal with Microsoft, Sun has been uncompromisingly determined to block the interoperability the marketplace demands.
If Micrsoft were to join the OASIS ODF TC today, seeking to adapt ODF to meet the legacy document-MSOffice features-line of business integration needs of their monopoly base, the TC would have to deal with the exact same issues as they have summarily rejected with current compatibility-interoeprability-convergence disussions!
There is no possible way anyone can claim that today's OASIS ODF TC would welcome Microsoft and make accomodating changes to the specification! No way! And the proof of this hostility can be seen in the actual disussions and rejections of Micrsoft specific interoperability proposals.
Both formats are valuable, and if ISO agrees to approve Open XML, we'll have the ISO guaranteeing that we'll always have access to the documentation.