I've been trying to make this work today and have hit what I suspected might be a critical problem. My environment is eclipse, GWT 2.3, GAE SDK 1.5. I wrote a simple servlet that takes no input, builds a new docx using parts of the supplied samples and then attempts to 'save' the output using some code found elsewhere on this forum like this:
SaveToZipFile saver = new SaveToZipFile(wordMLPackage);
try {
saver.save( response.getOutputStream() );
} ...