The next version of Office (for Windows and Macs) will support and XML file format as the default. Scoble has an interview with Brian Jones at Channel 9 that's worth listening to. Brian says that the majority of corrupt files that get sent to the Office team for review have been corrupted by third party applications. This would make life easier for those third party applications. What's more, new applications will be easier to write. Dan Farber also has some additional information, including how to get a preview, at Between the Lines.