tbody valign= not working in 1.72.0?

I'm using XSL version 1.72.0.

I have a <table> with markup starting as follows:

<informaltable pgwide="1" colsep="1" frame="all" rowsep="1">
        <tgroup cols="3">
          <colspec colname="c1" colwidth="1*"/>
          <colspec colname="c2" colwidth="2.5*"/>
          <colspec colname="c3" colwidth="2.5*"/>
          <thead>
            <row>
              <entry align="center"></entry>
              <entry align="center">AS</entry>
              <entry align="center">CPM</entry>
            </row>
          </thead>
         
          <tbody>
           
            <row>
              <entry>Putting components together</entry>
   etc.

Bob Stayton's book says ( 
http://www.sagehill.net/docbookxsl/CellAlignment.html#TableVertAlign )
I should be able to put the valign= attribute :

The |valign| attribute can be set on any of the following elements so it 
applies to the scope of that element:

thead
tbody
tfoot
row

It works to set it on "row", but doesn't work to set it on "tbody".

I have in my custom xsl the following: 

   <xsl:param name="use.extensions">1</xsl:param>
    <!-- next set to 0 for now.  1 requires xalan processor installed 
      see: http://sourceware.org/ml/docbook-apps/2004-q4/msg00703.html -->
   <xsl:param name="tablecolumns.extension">0</xsl:param> 

I looked in the XSL for 1.72.0, and see in common/table.xsl 
in the template 

<xsl:template name="inherited.table.attribute"> 

that the XSL does seem to be missing the code to look in tbody.

Is this a known problem, or am I interpreting things incorrectly?

-Marshall Schor