Inevitably I got that slightly wrong, since we want to output the running
total for each item, not just the final result. It should be

  <xsl:iterate select="input">
    <xsl:param name="total-so-far" select="0"/>
    <xsl:variable name="next-total" select="$total-so-far + ."/>
    <xsl:sequence select="$next-total"/>
    <xsl:next-iteration>     
        <xsl:with-param name="total-so-far" select="$next-total"/>
    </xsl:next-iteration>
  </xsl:iterate>

Regards,

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