Give us your feedback on the design of the file format. While the number one priority with the new formats will always be that the experience for the average end user doesn't change, I also care a lot about how useful they are to developers. I would love to hear feedback on what is difficult to do with the existing XML schemas for Word 2003. Maybe there are some areas we can look at changing.
Have you looked at the example file I posted the other week, or the developer whitepaper? Let me know what you'd like to see in the new format as well as what you think is problematic in the Office 2003 schemas. There is a public newsgroup (microsoft.public.office.xml) that's been around for awhile that is a great place to have discussions around this. Here is a pointer: http://msdn.microsoft.com/newsgroups/default.aspx?dg=microsoft.public.office.xml&lang=en&cr=US
If you have any feedback please comment either on this blog or go to the newsgroup. We've talked with a lot of customers about the designs of these formats, and we announced them really early so that we could get as much feedback as possible.