(C#) coroutines keep blocking GUI :?

I have a problem... I need to use WWW class to fetch some data from the server and then it should be displayed for user in the GUI.

What I though of is this:

static method that

 ...does some initialization, 

     var www = new WWW(url);

     // do stupid busy waiting till the server replies or an error occurs
     while (www.isDone != true)
     {}
 ... parses returned json or fails gracefully

then from GUI,

     public IList<ListEntity> entities = null; 
     void OnGUI()
     {
         if (entities != null)
         {
              // traverse the list and display data
         }
         if (GUI.Button(new Rect(140, 10, 250, 30), "Test "))
         {
             StartCoroutine(TestStuff());
         }
     }

and the fetcher part that sets the entities list so it could be displayed :

     IEnumerator TestStuff()
     {
         Debug.Log("Starting coroutine");
         entities = <call to static method>;
         Debug.Log("finished");
         yield return null;
     }

The problem is that, as far as I understand coroutines, it should be something similar to creating a thread. But unity does not behave this way. It halts the button press till the data is returned from the server.

Any ideas what I am doing wrong?

And is it possible to use normal threads in unity?

• UPDATE --
  Hmm, just tried moving the WWW part inside the GUI class. It does not block GUI now, when I can do

    var www = new WWW(url);
      yield return www;
  
  

I guess that will have to do...