> I'm trying to set an opening tag attribute based on the value 
> of my input; simple enough except that my closing tag needs
> to be outside of my select template.

Nope, it needs to be inside the template :)

Usually this question is asked in regards to constructing HTML tables, when
one wants to end a row with </tr> and start a new one with <tr> after having
generated some number of <td>...</td> cells in between.

Stop thinking about tags and start thinking about nodes.

The answer in your case is easy. Try this:

<xsl:template match="/">
  <Transaction>
    <EnrollmentRequest>
       <xsl:attribute name="servicetype">
         <xsl:choose>
           <xsl:when test="Transaction/LIN/LIN03='EL'">ELECTRIC</xsl:when>
           <xsl:otherwise>GAS</xsl:otherwise>
         </xsl:choose>
       </xsl:attribute>
       <!-- ... other stuff ... -->
    </EnrollmentRequest>
  </Transaction>
</xsl:template>
 


 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list