no-ip and Arduino Ethernet Shield

ok, I finally got a few minutes to do a quick test.

This code works (you need to replace the hostname and the encoded username:password with your values of course)

/*
  Web client
 
 This sketch connects to a website (http://www.google.com)
 using an Arduino Wiznet Ethernet shield. 
 
 Circuit:
 * Ethernet shield attached to pins 10, 11, 12, 13
 
 created 18 Dec 2009
 by David A. Mellis
 
modified for no-ip client example
by Jerry Sy aka doughboy
 */

#include <SPI.h>
#include <Ethernet.h>

// Enter a MAC address for your controller below.
// Newer Ethernet shields have a MAC address printed on a sticker on the shield
byte mac[] = {  0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };

// Initialize the Ethernet client library
// with the IP address and port of the server 
// that you want to connect to (port 80 is default for HTTP):
EthernetClient client;

void setup() {
  // start the serial library:
  Serial.begin(9600);
  // start the Ethernet connection:
  if (Ethernet.begin(mac) == 0) {
    Serial.println("Failed to configure Ethernet using DHCP");
    // no point in carrying on, so do nothing forevermore:
    for(;;)
      ;
  }
  // give the Ethernet shield a second to initialize:
  delay(1000);
  Serial.println("connecting...");

  // if you get a connection, report back via serial:
  if (client.connect("dynupdate.no-ip.com", 80)) {
    Serial.println("connected");
    // Make a HTTP request:
    //replace yourhost.no-ip.org with your no-ip hostname
    client.println("GET /nic/update?hostname=yourhhost.no-ip.org HTTP/1.0");
    client.println("Host: dynupdate.no-ip.com");
    //encode your username:password (make sure colon is between username and password) 
    //to base64 at http://www.opinionatedgeek.com/dotnet/tools/base64encode/
    //and replace the string below after Basic with your encoded string
    //clear text username:password is not accepted
    client.println("Authorization: Basic dXNlcm5hbWU6cGFzc3dvcmQ="); 
    client.println("User-Agent: Arduino Sketch/1.0 user@host.com");
    client.println();
  } 
  else {
    // if you didn't get a connection to the server:
    Serial.println("connection failed");
  }
}

void loop()
{
  // if there are incoming bytes available 
  // from the server, read them and print them:
  if (client.available()) {
    char c = client.read();
    Serial.print(c);
  }

  // if the server's disconnected, stop the client:
  if (!client.connected()) {
    Serial.println();
    Serial.println("disconnecting.");
    client.stop();

    // do nothing forevermore:
    for(;;)
      ;
  }
}

this is my serial monitor output (not my real ip address), result is nochg, meaning my ip address did not change

connecting...
connected
HTTP/1.1 200 OK
Date: Sat, 10 Mar 2012 01:37:40 GMT
Server: Apache/2
Content-Location: update.php
Vary: negotiate
TCN: choice
Content-Length: 20
Connection: close
Content-Type: text/plain; charset=UTF-8

nochg 50.131.456.123
disconnecting.

FWIW, un-encoded clear text password does not work.

Hope this helps.
Jerry