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Here's how I just repaired my .pages document for which Pages '09 proclaimed:
The required index.xml file is missing
1. Rename MyDocument.pages to MyDocument.zip
2. Uncompress MyDocument.zip by double-clicking it in the Finder
3. This results in a folder named MyDocument with NO extension
4. Add the .pages extension onto the folder filename: MyDocument.pages
5. Double-click MyDocument.pages in the Finder. It opens!
Not sure what's going on here, but it looks like Pages sometimes chokes on the auto-uncompress of the file into a folder when opening it. This has been an ongoing problem with Pages since its inception.
Hope this helps others stuck in this situation. Just saved myself a week of work! ;-)