I did figure something out.  Here is what I did:

               dim doc as XmlDocument = new XmlDocument()
               doc.Load(Server.MapPath("web.sitemap"))

               dim nodeList as XmlNodeList
               dim root as XmlNode = doc.DocumentElement

               dim nsmgr as XmlNamespaceManager = new
XmlNamespaceManager(doc.NameTable)
               nsmgr.AddNamespace("ns0",
"xmlns:ns0='urn:schemas-microsoft-com:xslt")

               dim nodeLevel as string = ""
               dim i as integer
               for i = 1 to 4
                   nodeLevel = nodeLevel & "siteMapNode/"
                   dim xmlString as string = nodeLevel &
"siteMapNode[contains(translate(@title,'abcdefghijklmnopqrstuvwxyz','ABCDEFGHIJKLMNOPQRSTUVWXYZ'),
'" + txtSearchCriteria.Text.ToUpper() + "')]"
                   'dim xmlString as string =
"siteMapNode/siteMapNode/siteMapNode/siteMapNode[contains(translate(@title,'abcdefghijklmnopqrstuvwxyz','ABCDEFGHIJKLMNOPQRSTUVWXYZ'),
'" + txtSearchCriteria.Text.ToUpper() + "')]"
                   nodeList=root.SelectNodes(xmlString,nsmgr)

                   'Change the price on the books.
                   dim book as XmlNode
                   for each book in nodeList
                       Dim tempRow As New TableRow()
                       Dim tempCell As New TableCell()
                       tempCell.Text = "<a class='HoverStyle' href=" &
book.Attributes("url").Value & " target='_blank' a>" &
book.Attributes("title").Value & "</a>"
                       tempRow.Cells.Add(tempCell)
                       tblSearchResults.Rows.Add(tempRow)
                   next
               next i

It loops four levels of the xml doc.  Although I would like to be able
to determone the number of levels without hard-coding.  I will have to
research this.

Then you need the XPath expression
 //siteMapNode
to search from the document root node or
 .//siteMapNode
to search from the context node.