Sorry David, forgot about the system-property,  so your example would
then look like:

<xsl:if test="system-property('xsl:vendor')='Microsoft'">
         <xsl:comment><![CDATA[[if IE]><![if lt IE
7]><![endif]]]></xsl:comment>
          <script src="/scripts/ie5.js" type="text/javascript">//*
  imported via src attribute *//</script>
          <style type="text/css">
            @import "/browser/ie5/base.css";
          </style>
         <xsl:comment><![CDATA[[if IE]><![endif]><![endif]]]></xsl:comment>
</xsl:if>

Manfred