Sorry,if I am mistaken: I not so well sign with xml, but I wish to share some problem and its decision.
Moodle 1.5 use XHTML, but content type generated pages is text/html. It creates problems with XHTML namespaces:
If I try to use such code
It does not work.
And, if change content type generated pages to application/xhtml+xml - all marking of page spoils
But, if change content type to application/xhtml+xml, and add atribute xmlns="http://www.w3.org/1999/xhtml" in tag <html> - the marking of page and formula MathML are displayed correctly
P.S. I use .xml extension thet for .xhtml content type in this site not defined .