I would like to encourage you to have a look at frevvo, a Web start-up launched by some friends, that provides a full solution to handle complex Web 2.0 forms, right from your browser. One of their strong selling points is the great form designer available as a Web application (AJAX based).
Also, the form can be directly mapped to an XML Schema that you have imported. Their server component will directly generate valid XML instances when you submit the form to it. Currently it is based on a Servlet container but they are experimenting with Restlet (and already provided key feed-back), so I’m hoping that they will migrate when our final 1.0 version comes out 😉