On the MSDN site there is an example of some C# code that shows how to make a web request with POST'ed data. Here is an excerpt of that code:

WebRequest request = WebRequest.Create ("http://www.contoso.com/PostAccepter.aspx ");
request.Method = "POST";
string postData = "This is a test that posts this string to a Web server.";
byte[] byteArray = Encoding.UTF8.GetBytes (postData); // (*)
request.ContentType = "application/x-www-form-urlencoded";
request.ContentLength = byteArray.Length;
Stream dataStream = request.GetRequestStream ();
dataStream.Write (byteArray, 0, byteArray.Length);
dataStream.Close ();
WebResponse response = request.GetResponse ();
...more...

The line marked (*) is the line that puzzles me. Shouldn't the data be encoded using the UrlEncode function than UTF8? Isn't that what application/x-www-form-urlencoded implies?