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Thanks Dave B, 
I feel like I'm almost there.
I've got the following code running, and I'm getting to the post now to update an existing user's email. And I believe the xml is set up correctly now.  
However, when I post the user's xml, it doesn't get any response and the user doesn't get added to my list.  Any thoughts when looking at the following code?
 
// get the configuration for constant contact as well as any params
    private readonly string APIKey = ConfigurationManager.AppSettings["ConstantContactKey"];
    private readonly string Password = ConfigurationManager.AppSettings["ConstantContactPass"];
    private readonly string Username = ConfigurationManager.AppSettings["ConstantContactUser"];
    private readonly string first = Utility.GetStringFromRequest("newsletter-first");
    private readonly string last = Utility.GetStringFromRequest("newsletter-last");
    private readonly string list = Utility.GetStringFromRequest("subscriber-type");

    private string APIURI = "https://api.constantcontact.com/ws/customers/{0}";
    private string ContactURI = "";

    private string email = Utility.GetStringFromRequest("newsletter-email").ToLower();
    private string ListURI = "http://api.constantcontact.com/ws/customers/{0}/lists/{1}";
    private XmlDocument doc;

    protected void Page_Load(object sender, EventArgs e)
    {
        email = Server.UrlEncode(email);

        APIURI = String.Format(APIURI, Username);
        ContactURI = APIURI + "/contacts";
        ListURI = String.Format(ListURI, Username, list);
        
        // get the user data
        string xml = Get();
        doc = new XmlDocument();
        doc.LoadXml(xml);

        XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
        nsmgr.AddNamespace("a", "http://www.w3.org/2005/Atom");

        // check for an entry
        XmlNode link = doc.SelectSingleNode("//a:entry/a:link", nsmgr);
        if (link == null)
        {
            Response.Write(Post());
        }
        else
        {
            // Get the list
            ContactURI = "https://api.constantcontact.com" + link.Attributes["href"].Value;
            xml = GetLists();

            doc = new XmlDocument();
            doc.LoadXml(xml);

            nsmgr = new XmlNamespaceManager(doc.NameTable);
            nsmgr.AddNamespace("cc", "http://ws.constantcontact.com/ns/1.0/");

            // append the xml returned by the get
            XmlNode lists = doc.SelectSingleNode("//cc:Contact/cc:ContactLists", nsmgr);
            XmlNode newList = doc.CreateElement("ContactList");
            XmlAttribute att = doc.CreateAttribute("id");
            att.Value = ListURI;
            newList.Attributes.Append(att);
            
            newList.InnerXml = "<link xmlns=\"http://www.w3.org/2005/Atom\" href=\"" + ListURI + "\" rel=\"self\"
><OptInSource>ACTION_BY_CUSTOMER</OptInSource><OptInTime>" + DateTime.Now.ToUniversalTime().ToString("yyyy-MM-dd\\THH:mm:ss\\Z") +
</OptInTime>";

            lists.AppendChild(newList);
            
            // write the posted xml to a file for confirmation of xml format
            doc.PreserveWhitespace = true;
            XmlTextWriter wrtr = new XmlTextWriter(Server.MapPath("/test.xml"), System.Text.Encoding.Unicode);
            doc.WriteTo(wrtr);
            wrtr.Close();

            Response.Write(Put());
        }
    }

    private string Get()
    {
        CredentialCache LoginCredentials = new CredentialCache();
        LoginCredentials.Add(new Uri(APIURI), "Basic", new NetworkCredential(APIKey + "%" + Username, Password));
        WebRequest Request = WebRequest.Create(ContactURI + "?email=" + email); // URI for the GET

        Request.Credentials = LoginCredentials;
        Request.Method = "GET";
        Request.ContentType = "application/atom+xml";

        try
        {
            HttpWebResponse Response = (HttpWebResponse) Request.GetResponse();
            StreamReader Reader = new StreamReader(Response.GetResponseStream());
            string XMLResponse = Reader.ReadToEnd();

            Reader.Close();
            Response.Close();

            return XMLResponse;
        }
        catch (WebException e)
        {
            return "WebException: " + e.Status + " " + e.Message;
        }
        catch (Exception e)
        {
            return "Exception:" + e.Message;
        }
    }

    private string GetLists()
    {
        CredentialCache LoginCredentials = new CredentialCache();
        LoginCredentials.Add(new Uri(APIURI), "Basic", new NetworkCredential(APIKey + "%" + Username, Password));
        WebRequest Request = WebRequest.Create(ContactURI);

        Request.Credentials = LoginCredentials;
        Request.Method = "GET";
        Request.ContentType = "application/atom+xml";

        try
        {
            HttpWebResponse Response = (HttpWebResponse) Request.GetResponse();
            StreamReader Reader = new StreamReader(Response.GetResponseStream());
            string XMLResponse = Reader.ReadToEnd();

            Reader.Close();
            Response.Close();

            return XMLResponse;
        }
        catch (WebException e)
        {
            return "WebException: " + e.Status + " " + e.Message.ToString();
        }
        catch (Exception e)
        {
            return "Exception:" + e.Message;
        }
    }

    protected string Put()
    {
        CredentialCache LoginCredentials = new CredentialCache();
        LoginCredentials.Add(new Uri(APIURI), "Basic", new NetworkCredential(APIKey + "%" + Username, Password));
        HttpWebRequest Request = (HttpWebRequest) WebRequest.Create(ContactURI);

        Request.Method = "PUT";
        Request.ContentType = "application/atom+xml";
        Request.Credentials = LoginCredentials;

        string XMLData = doc.OuterXml;

        byte[] byteArray = Encoding.UTF8.GetBytes(XMLData);
        try
        {
            Request.ContentLength = byteArray.Length;
            string XMLResponse = "Bytes to send: " + byteArray.Length;
            Stream streamRequest = Request.GetRequestStream();

            streamRequest.Write(byteArray, 0, byteArray.Length);
            streamRequest.Close();

            HttpWebResponse Response = (HttpWebResponse) Request.GetResponse();
            StreamReader Reader = new StreamReader(Response.GetResponseStream());

            XMLResponse += Reader.ReadToEnd();

            Reader.Close();
            Response.Close();

            return XMLResponse;
        }
        catch (WebException e)
        {
            return "WebException: " + e.Status + " With response: " + e.Message;
        }
        catch (Exception e)
        {
            return "Exception:" + e.Message;
        }
    }
    private string Post()
   {
        CredentialCache LoginCredentials = new CredentialCache();
        LoginCredentials.Add(new Uri(APIURI), "Basic", new NetworkCredential(APIKey + "%" + Username, Password));
        HttpWebRequest Request = (HttpWebRequest) WebRequest.Create(ContactURI);
        Request.Method = "POST";
        Request.ContentType = "application/atom+xml";
        Request.Credentials = LoginCredentials;
        string xmldata = BuildXML();

        byte[] byteArray = Encoding.UTF8.GetBytes(xmldata);

        try
        {
            Request.ContentLength = byteArray.Length;
            string XMLResponse = "Bytes to send: " + byteArray.Length;

            Stream streamRequest = Request.GetRequestStream();

            streamRequest.Write(byteArray, 0, byteArray.Length);
            streamRequest.Close();
            HttpWebResponse response = (HttpWebResponse) Request.GetResponse();
            StreamReader Reader = new StreamReader(response.GetResponseStream());
            XMLResponse += response.StatusCode + " " + response.StatusDescription + " " + Reader.ReadToEnd();

            Reader.Close();
            response.Close();
            return "{success:1,message:'Thank you for subscribing.'}";
        }
        catch (WebException e)
        {
            Utility.Log(String.Format("CC Exception: first: {0}, last: {1}, email: {2}, list: {3}, webException: {4} with response: {5}", first, last, email, list, e.Status, e.Message));
            return "{success:0,message:'<br/>Your email has been saved and will be added at a later time.<br/>" + e.Message + "'}";
        }
        catch (Exception e)
        {
            Utility.Log(String.Format("CC Exception: first: {0}, last: {1}, email: {2}, list: {3}, error: {4}", first, last, email, list, e));
            return "{success:0,message:'<br/>Your email has been saved and will be added at a later time.<br/>" + e.Message + "'}";
        }
    }
    protected string BuildXML()
    {
        StringBuilder XMLData = new StringBuilder();
        XMLData.Append("<entry xmlns=\"http://www.w3.org/2005/Atom\">");
        XMLData.Append("<title type=\"text\"> </title>");
        XMLData.Append("<updated>" + DateTime.Now + "</updated>");
        XMLData.Append("<author>IFC</author>");
        XMLData.Append("<id>data:,none</id>");
        XMLData.Append("<summary type=\"text\">Contact</summary>");
        XMLData.Append("<content type=\"application/vnd.ctct+xml\">");
        XMLData.Append("<Contact xmlns=\"http://ws.constantcontact.com/ns/1.0/\">");
        XMLData.Append("<EmailAddress>" + email + "</EmailAddress>");
        XMLData.Append("<FirstName>" + first + "</FirstName>");
        XMLData.Append("<LastName>" + last + "</LastName>");
        XMLData.Append("<OptInSource>ACTION_BY_CONTACT</OptInSource>");
        XMLData.Append("<ContactLists>");
        XMLData.Append("<ContactList id=\"" + ListURI + "\" />");
        XMLData.Append("</ContactLists>");
        XMLData.Append("</Contact>");
        XMLData.Append("</content>");
        XMLData.Append("</entry>");
        return XMLData.ToString();
    }
 
Thanks for any help you can give.