I just wrote the following code and ran it in the iPhone simulator on my mac using MonoDevelop and MonoTouch.  With this code, I wrote it in .NET and then opened it in MonoDevelop, massaged appropiately, and then built it.  It ran!  Very cool.  Here's a copy of the code from VS.NET.  It ran perfect in the MonoDevelop IDE and the iPhone simulator.  A word of warning, running in the simulator is no guarantee it will run in a physical iPhone or iPod Touch.

Code:

            string Url = "http://somewhere";
            string Body = "{\"ld\":{\"UserName\":\"blah\",\"PassWord\":\"blah\",\"AppKey\":\"blah\"}}";
            byte[] byteData = UTF8Encoding.UTF8.GetBytes(Body);
            try
            {
                // Create the web request
                HttpWebRequest request = WebRequest.Create(Url) as HttpWebRequest;
                request.ContentLength = Body.Length;

                // Set type to POST
                request.Method = "POST";
                request.ContentType = "text/json";
               
                // Write the parameters
                StreamWriter stOut = new StreamWriter(request.GetRequestStream(), System.Text.Encoding.ASCII);
                stOut.Write(Body);
                stOut.Close();
              
                request.BeginGetResponse(new AsyncCallback(ProcessHttpResponse), request);
            }
            catch (WebException we)
            {
                Console.Error.WriteLine("Exception: " + we.Message);
            }
            catch (System.Exception sysExc)
            {
                Console.Error.WriteLine("Exception: " + sysExc.Message);
            }
 

Here is the callback:

        private void ProcessHttpResponse(IAsyncResult iar)
        {
            HttpWebRequest request = (HttpWebRequest)iar.AsyncState;
            HttpWebResponse response;
            response = (HttpWebResponse)request.EndGetResponse(iar);
            Console.Error.WriteLine("get response.");
            System.IO.StreamReader strm = new System.IO.StreamReader(
                response.GetResponseStream());
            string responseString = strm.ReadToEnd();
            response.Close();
            Console.Error.WriteLine("response: " + responseString);
        }
 

Using statements from my MonoTouch app:

using System;
using System.Collections.Generic;
using System.IO;
using System.Net;
using System.Linq;
using System.Text;
using System.Xml;
using MonoTouch.Foundation;
using MonoTouch.UIKit;