How to send Json to a webservice from pc/RPI with live data from Arduino(serial)
I'm kind of tackling the same thing at the moment.  

I have been using MQTT for a while for things like this. Including on an Netduino, an Arduino, and today a LEGO NXT.

But it is TCP based. I have been wanting to use serial, not TCP (ethernet) for a while, and when the need came for the NXT, it was time to write my own bridge to existing code.

I runs on the same host as the MQTT broker, but listens to a serial port, and mirrors the data to a TCP MQTT broker.

The code is turning out to be much simpler than I expected, I should have done this months ago.

Nxt Side

while (true)
{
    float x, y, z;
    DIMUreadGyroAxes(IMU, x, y, z);

    char json[128];
    sprintf(json, "PUBnxt/Gyro,{\"x\":%f,\"y\":%f,\"z\":%f}\n", x, y, z);
    nxtWriteRawHS(json, strlen(json), 0); // wireless serial over xbee
    wait1Msec(50);
}


PC side

 void Serial_DataReceived(object sender, SerialDataReceivedEventArgs e)
    {
        SerialPort p = sender as SerialPort;
        while (p.BytesToRead &gt; 0)
        {
            string line;
            try
            {
                line = p.ReadLine();
            }
            catch (IOException)
            {
                throw;  // +++timeout
            }
            // parse it, it should be CMDtopic[,payload]
            if (line.Substring(0, 3).Equals("PUB"))
            {
                int commaIdx = line.IndexOf(',', 3);
                string topic = line.Substring(3, commaIdx - 3);
                string payload = line.Substring(commaIdx + 1);
                Console.WriteLine("{3}: {0}, publish topic({1}) payload({2})", p.PortName, topic, payload, DateTime.Now.ToLongTimeString());
                Mqtt.Publish(topic, System.Text.Encoding.UTF8.GetBytes(payload));

            }
            else if (line.Substring(0, 3).Equals("SUB"))
            {
                string topic = line.Substring(3);
                Console.WriteLine("{0}, subscribe topic({1})", p.PortName, topic);
                throw new NotImplementedException("MQTT Subscribe not implemented yet");
            }
            else
            {
                //Debugger.Break();   // ignore it, framing error
                Console.WriteLine("{0}, Data dropped", p.PortName);
            }
        }
    }


You might also want to look at data.sparkfun.com. A similar, complete concept, already existing.