I saw a paper from a commercial vendor at:
http://www.deltaxml.com/pdf/merging-xml-files.pdf
I would be very interested to learn what you find out.
I've played around with the deltaxml output and though I didn't have any
use for it at the time I was pleased, it gave the impression of a
product upon which one could build any number of xml differencing
systems.
I wonder, has anybody played around with subversion to get back well
formed
diffs?
I think Jeni has some posted examples of this before.
XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list