Jesse asks how to insert editable areas into the page. I'm not sure if you know this, but you can put an editable="true" attribute on any HTML element in IE and it becomes a WYSIWYG text box.
That being said - we worked on a browser-based XML document editor a year or two back, which sounds like what you're talking about. Basically, it took an XSD schema and built a corresponding HTML page for it. The HTML page could add new sections for repeating nodes and so on. It was pretty neat, though it's pretty fragile to do in the browser in general, and it only worked at all in IE.
(Hmm.... maybe that's why I'm not as impressed with InfoPath as everyone else seems to be, but....)
Still, if anyone is interested - I'll see if I can whip up something similar in a server control.