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I wish to control a relay through emoncms.org from a web page located on my Android mobile phone.
To do this, I had thought of creating a feed. So, I can update the value of the feed through the website or web page located in my phone or in a PC. In my house, an Arduino with Ethernet module must be able to read the feed and depending on the values ​​received, switch on or off the relay.
I have created the input and the corresponding feed. From a browser I can give values to the feed. But the problem is that I don't know how Arduino can read the last value of that feed. 

I know that sentence is like: http://emoncms.org/feed/value.json?id=32427&apikey=xxxxxxxxxxx

But how to transfer this into the arduino with the GET method?

I've tried this (Arduino web client sample modification):

#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 };
// if you don't want to use DNS (and reduce your sketch size)
// use the numeric IP instead of the name for the server:

char server[] = "www.emoncms.org";    // name address for Google (using DNS)
char apikey[] = "myapikey";

// Set the static IP address to use if the DHCP fails to assign
IPAddress ip(192,168,0,177);

// 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() {
// Open serial communications and wait for port to open:
  Serial.begin(9600);
   while (!Serial) {
    ; // wait for serial port to connect. Needed for Leonardo only
  }

  // 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:
    // try to congifure using IP address instead of DHCP:
    Ethernet.begin(mac, ip);
  }
  // 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(server, 80)) {
    Serial.println("connected");
    // Make a HTTP request:
   
    client.print("GET /feed/value.json?apikey=");
    client.print(apikey);
    client.print("?id=32427");
    client.println(" HTTP/1.1");
    
    client.println("Host:emoncms.org");
    client.println("User-Agent: Arduino-ethernet");
    client.println("Connection: close");
    client.println();
  }
  }
  else {
    // kf 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:
    while(true);
  }
}

But the response is:

connecting...
connected
HTTP/1.1 200 OK
Date: Tue, 04 Mar 2014 21:37:29 GMT
Server: Apache/2.2.22 (Ubuntu)
X-Powered-By: PHP/5.3.10-1ubuntu3.9
Vary: Accept-Encoding
Content-Length: 49
Connection: close
Content-Type: text/html

{"success":false,"message":"Feed does not exist"}
disconnecting.

I´ve tried with another id feed and is the same return...

Can anyone help me ?

Thanks.