Untitled

    var test = document.createElement("p");
    test.setAttribute("id","testing");
    document.getElementById("body_main").appendChild(test);

    new Ajax.Request("test.php",
                     {
                         onSuccess: function(response)
                         {
                          document.getElementById("testing").setAttribute("value","testing...");
                         },
                         onFailure: function(response)
                         {
                             alert("fail");
                         }
                     });

        alert(document.getElementById("testing").getAttribute("value"));