Nevermind... sorted.  Each if-endif needs to have its own xsl:comment,
instead of stuffing everything in one xsl:comment.  The following syntax
works nicely:

<xsl:comment>[if IE]<![CDATA[ ... ]]>&lt;![endif]</xsl:comment>

Using CDATA depends on the content, it isn't required.  But, if it is
used, it needs to be inside the if-endif.

-Eric