Regarding my solution, this syntax should have come to my mind..
<xsl:element name="US_{local-name()}">

I would do this as:

 <xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xsl:output method="xml" indent="yes" />

  <xsl:template match="*">
    <xsl:copy>
      <xsl:apply-templates />
    </xsl:copy>
  </xsl:template>

  <xsl:template match="address/*">
    <xsl:element name="US_{local-name()}">
      <xsl:apply-templates/>
    </xsl:element>
  </xsl:template>

</xsl:stylesheet>

 Michael Kay
http://www.saxonica.com/

--
Regards,
Mukul Gandhi