Hi all,
Ondrej Tichy just point this editor out to me as a potential for crowd sourcing structural work: http://cvsdemo.bitfluxeditor.org/examples/uni/index.html
Looks quite interesting. You change modes by clicking on the little arrows in the left hand corner. In tag mode, you get a list of possible tags at each insertion point by right clicking. When you try to save XML that doesn't work with schema or is malformed, you get an error.
I'm not sure this is ready for naive users, but it is pretty good. It might be useful for teams with collaborating experienced XMLers. Though, maybe not over a svn+local editing system (sorry Ondrej ;) ).
-dan