Many thanks to Santiago Pericas-Geertsen, who blogged about this exact problem earlier this year. If you can't figure out for the life of you why your document won't validate, even though you know it's valid, try setting setNamespaceAware(true) on your DocumentBuilderFactory:
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
factory.setNamespaceAware(true);
Chances are that's all you need and you'll be up and running.
Programming is easy, it's all the gotchas that make it hard.
