Using Jeni Tennisons Markup Utility on XHTML

I'm trying to use Jeni Tennison's Markup Utility to mark up glossary terms
in an XML document as part of a transformation to HTML pages. Some of the
text elements contain XHTML, and I can't figure out how to apply the Markup
Utility on these elements without loosing the XHTML tags embedded in the
text. For examlpe if I have an XML element <ns1:Text>:
 
<ns1:Text><div class="RTF" xmlns="http://www.w3.org/1999/xhtml">Text with
glossary <b>terms</b> to be marked up</div></ns1:Text>
 
I would normally render this element to the HTML output using <xsl:copy-of>,
but the when applying the Markup Utility, I'll loose all the XHTML tags (and
thus the text formatting) in the output because the Markup Utility renders
using <xsl:value-of> .
 
Does anyone know of a simple solution to this?