>   ...actually, I found that Saxon 6.5.5 fails here, reporting 
> a compilation error:
> 

What exactly did you do? The following compiles and executes successfully
under Saxon 6.5.5:

<xsl:stylesheet
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0">
    
    <!-- Test xsl:namespace guarded by element-available()  -->
   
    <xsl:template match="/">
       
			<a>
			  <xsl:if test="element-available('xsl:namespace')">
				  <xsl:namespace name="pre">
				     <xsl:sequence select="'uri'"/>
				  </xsl:namespace>
			  </xsl:if>  
		      </a>
               
    </xsl:template>
</xsl:stylesheet>

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