Global.asax

public class WebApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        RouteTable.Routes.MapRoute("HttpProxy", "proxy/{*path}", new { controller = "Proxy", action = "Http" })
    }
}

init.coffee

window.su = {} # declare app namespace

#We only need proxy for IE as other browsers support CORS

su.initServerUrl = (url) ->
  if $.browser.msie && parseInt($.browser.version, 10) < 10
    su.serverUrl = '/proxy'
  else
    su.serverUrl = url  # e.g. http://api.mydataserver.com

ProxyController.cs

public class ProxyController : System.Web.Mvc.Controller
{
    protected string svcUrl = WebConfigurationManager.AppSettings["SvcUrl"];

    public ActionResult Http()
    {  
        return Content(grabContent(HttpContext.Request.Url.PathAndQuery.Replace("/proxy","")));
    }

    /// <see>http://stackoverflow.com/questions/3447589/copying-http-request-inputstream</see>
    private string grabContent(string url)
    {
        string content;

        // Create a request for the URL. 		
        var req = HttpWebRequest.Create(svcUrl+url);
        req.Method = HttpContext.Request.HttpMethod;

        //-- No need to copy input stream for GET (actually it would throw an exception)
        if (req.Method != "GET")
        {
            req.ContentType = "application/json";

            Request.InputStream.Position = 0;  //***** THIS IS REALLY IMPORTANT GOTCHA
                
            var requestStream = HttpContext.Request.InputStream;
            Stream webStream = null;
            try
            {
                //copy incoming request body to outgoing request
                if (requestStream != null && requestStream.Length > 0)
                {
                    req.ContentLength = requestStream.Length;
                    webStream = req.GetRequestStream();
                    requestStream.CopyTo(webStream);
                }
            }
            finally
            {
                if (null != webStream)
                {
                    webStream.Flush();
                    webStream.Close(); 
                }
            }
        }

        // If required by the server, set the credentials.
        req.Credentials = CredentialCache.DefaultCredentials;

        // No more ProtocolViolationException!
        using (HttpWebResponse response = (HttpWebResponse)req.GetResponse())
        {
            // Display the status.
            //Console.WriteLine(response.StatusDescription);
                
            // Get the stream containing content returned by the server.
            using (Stream dataStream = response.GetResponseStream())
            {
                // Open the stream using a StreamReader for easy access.
                StreamReader reader = new StreamReader(dataStream);
                // Read the content. 
                content = reader.ReadToEnd();
            }
        }

        return content;
    }
}