The title slide will be output for the [s:]authors element:

<xsl:template match="s:slides/s:authors">
<fo:block break-after="page" text-align="center" font-weight="bold">
<fo:block font-size="150%"><xsl:value-of select="preceding-sibling::s:title"/></fo:block>
<xsl:apply-templates/>
</fo:block>
</xsl:template>

The [fo:]block element acts a lot like XHTML's p element.