As I understnd it, Microsoft has only published interim descriptions for the
docx format, so
a. even if you created transforms to convert betweeen those and Word 2003
XML format, there is no guarantee that those transforms would work with the
production release of Office 2007
b. if you are a participant in an Office 2007 beta , the best place to look
for answers to this kind of question would be the appropriate beta
newsgroup. If you aren't a participant in the beta, I wouldn't expect to see
any information other than the material Microsoft chooses to publish on its
website.

Peter Jamieson

If I understand your question well, yes there is. You can save documents in
2007 using save as... Word 2003.
But I read somewhere that users of MS Word 2003 can update a tool with which
they also can read .docx documents.
Fred

Yes, as of May 23.  See
http://www.microsoft.com/office/preview/beta/converter.mspx .