Of course, the desired behavior was built right into the navigation prototype ;)

You need to check if the currentpage is same as that within the loop of for-each.

Here is modified code of your xslt. Hope this helps

<xsl:param name="currentPage"/>

    <!-- Input the documenttype you want here -->
    <xsl:variable name="level" select="1"/>

    <xsl:template match="/">

      <xsl:if test="count($currentPage/ancestor-or-self::* [@level=$level]/* [@isDoc and string(umbracoNaviHide) != '1']) &gt; 0">
 <ul id="topNavigation">
      <xsl:for-each select="$currentPage/ancestor-or-self::* [@level=$level]/* [@isDoc and string(umbracoNaviHide) != '1']">
      <xsl:choose>
      <xsl:when test="$currentPage/@id = ./@id">
      <li>

    <a href="{umbraco.library:NiceUrl(@id)}">

        <xsl:attribute name="class">current</xsl:attribute>

      <xsl:value-of select="@nodeName"/>
    </a>
  </li>
      </xsl:when>
      <xsl:otherwise>
      <li>

    <a href="{umbraco.library:NiceUrl(@id)}">
      <xsl:value-of select="@nodeName"/>
    </a>
  </li>
      </xsl:otherwise>
      </xsl:choose>

</xsl:for-each>
</ul>
</xsl:if>
    </xsl:template>

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