I think I figured it out!  Here's a sanitized overview of what I did:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
xmlns:msxsl="urn:schemas-microsoft-com:xslt" 
xmlns:var="http://schemas.microsoft.com/BizTalk/2003/var" version="1.0">
  <xsl:output omit-xml-declaration="yes" version="1.0" method="xml" />
  <xsl:key name="POKey" match="//LineItems/LineItem" 
use="concat(@FulfillmentPartnerId,'_',@DeliveryAddressId,'_',@DeliveryMethodId)" 
/>
  <xsl:template match="/">
    <PurchaseOrders>
      <xsl:apply-templates select="*" />
    </PurchaseOrders>
  </xsl:template>
  <xsl:template match="LineItem">
    <xsl:variable name="nodeList" select="key('POKey', 
concat(@FulfillmentPartnerId,'_',@DeliveryAddressId,'_',@DeliveryMethodId))"/>
    <xsl:if test="generate-id($nodeList) = generate-id()">
      <PurchaseOrder>
        <xsl:attribute name="Id">
          <xsl:value-of select="@Id" />
        </xsl:attribute>
        <xsl:attribute name="PartnerId">
          <xsl:value-of select="@FulfillmentPartnerId" />
        </xsl:attribute>
        <xsl:attribute name="DeliveryMethodId">
          <xsl:value-of select="@DeliveryMethodId" />
        </xsl:attribute>
        ...
        <xsl:element name="LineItems">
          <xsl:apply-templates select="$nodeList" mode="innerGroup" />
        </xsl:element>
        <xsl:element name="DeliveryAddress">
          <xsl:variable name="var:DeliveryAddressId" 
select="@DeliveryAddressId" />
          <xsl:apply-templates 
select="../../Addresses/Address[@Id=$var:DeliveryAddressId]" 
mode="innerGroup" />
        </xsl:element>
        ...
      </PurchaseOrder>
    </xsl:if>
  </xsl:template>
  <xsl:template match="LineItem" mode="innerGroup">
    <LineItem>
      <xsl:attribute name="Id">
        <xsl:value-of select="@Id" />
      </xsl:attribute>
      <xsl:attribute name="ProductId">
        <xsl:value-of select="@ProductId" />
      </xsl:attribute>
      <!-- Using a BizTalk Server 2006 Extension Object -->
      <xsl:variable name="var:v3" 
select="ScriptNS0:RetrievePartnerProductId(string(@FulfillmentPartnerId) , 
string(@ProductId))" />
      <xsl:attribute name="FulfillmentPartnerProductId">
        <xsl:value-of select="$var:v3" />
      </xsl:attribute>
      <xsl:attribute name="Quantity">
        <xsl:value-of select="@Quantity" />
      </xsl:attribute>
      <xsl:attribute name="UnitPrice">
        <xsl:value-of select="@UnitPrice" />
      </xsl:attribute>
      ...
    </LineItem>
  </xsl:template>
  <xsl:template match="Address" mode="innerGroup">
    <xsl:attribute name="FirstName">
      <xsl:value-of select="@FirstName" />
    </xsl:attribute>
    ...
  </xsl:template>
</xsl:stylesheet>

Cheers,
Colin

"Colin Bowern" <colin.bowern@n...> wrote in message 
news:utI4zM7zGHA.4648@T......
>I have a document where I want to group based on values several levels 
>deep:
>
> <SalesOrder ...>
>    <LineItems>
>        <LineItem Id="1" DeliveryAddressId="Shipping" DeliveryMethodId="1" 
> SupplierId="A" />
>        <LineItem Id="2" DeliveryAddressId="Shipping" DeliveryMethodId="2" 
> SupplierId="A" />
>        <LineItem Id="3" DeliveryAddressId="Shipping" DeliveryMethodId="1" 
> SupplierId="B" />
>        <LineItem Id="4" DeliveryAddressId="Billing" DeliveryMethodId="2" 
> SupplierId="B" />
>    </LineItems>
> </SalesOrder>
>
> The above example should ideally end up in four purchase orders because of 
> the combination of delivery address, method, and supplier.  For example:
>
> <PurchaseOrder SupplierId="A" DeliveryMethod="1">
>    <DeliveryAddress Id="Shipping" .. />
>    <LineItems>
>        <LineItem Id="1" />
> </PurchaseOrder>
>
> At this point I've gotten to a point where I can build out single 
> line-item purchase orders based on this information.   Ideally I want to 
> aggregate based on the three values using XSLT.
>
> Has anyone seen this scenario before?  Are there any good resources that 
> you could share to help me understand what functions/approaches to take to 
> resolve this?
>
> Thanks,
> Colin
>