bueno después de la espera ya tengo el nuevo shield ethernet basado en w5100.
Os comento mas o menos lo que he echo.
una vez conectado y configurado, cargo el sketch de telnet client. y este es el resultado que me da

me salen caracteres raros y supuesta-mente me deberia de pedir el usuario y la clave.. ¿no?
bueno os subo unas fotos para que veáis como me conecto al mikrotik con telnet.
abro el telnet de windows desde cmd.

Despues introduzco usuario = admin

Despues introduzco la clave = (pulso intro por que no tiene)

y por ultimo le paso parámetros para configurarlo.

bueno os dejo el código de arduino pero es el mismo que el que trae las libreria..
/*
Telnet client
This sketch connects to a a telnet server (http://www.google.com)
using an Arduino Wiznet Ethernet shield. You'll need a telnet server
to test this with.
Processing's ChatServer example (part of the network library) works well,
running on port 10002. It can be found as part of the examples
in the Processing application, available at
http://processing.org/
Circuit:
* Ethernet shield attached to pins 10, 11, 12, 13
created 14 Sep 2010
by Tom Igoe
*/
#include <SPI.h>
#include <Ethernet.h>
// Enter a MAC address and IP address for your controller below.
// The IP address will be dependent on your local network:
byte mac[] = {
0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
byte ip[] = {
192,168,0,2 };
// Enter the IP address of the server you're connecting to:
byte server[] = {
192,168,0,1 };
// Initialize the Ethernet client library
// with the IP address and port of the server
// that you want to connect to (port 23 is default for telnet;
// if you're using Processing's ChatServer, use port 10002):
Client client(server, 23);
void setup() {
// start the Ethernet connection:
Ethernet.begin(mac, ip);
// start the serial library:
Serial.begin(9600);
// 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()) {
Serial.println("connected");
}
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);
}
// as long as there are bytes in the serial queue,
// read them and send them out the socket if it's open:
while (Serial.available() > 0) {
char inChar = Serial.read();
if (client.connected()) {
client.print(inChar);
}
}
// if the server's disconnected, stop the client:
if (!client.connected()) {
Serial.println();
Serial.println("disconnecting.");
client.stop();
// do nothing:
while(true);
}
}
Espero que este todo claro y que si se os ocurre el porque me ayudéis...