Haven't had a chance to check it out in detail but at first blush it looks very interesting indeed: http://www.callistocms.com/

• two years of research behind it
• was commercial, open source'd on 2002-July-01 (GPL)
• based on XSL stylesheets
• WYSIWYG XML editing - the screen shots look very exciting. The editing is fine grained and doesn't require a seperate page. E.g. only the element (paragraph) which is currently being edited is in a plaintext form, the rest of the page remains in html-view mode. Unfortunately the live demo seems to have been taken down (probably too much traffic). Edit-mode screenshot:

-- MattWilkie - 04 Jul 2002