You need a GUIStyle that has a centered alignment. You can do this programmatically by getting an existing style and setting the alignment:

function OnGUI () {
    var centeredStyle = GUI.skin.GetStyle("Label");
    centeredStyle.alignment = TextAnchor.UpperCenter;
    GUI.Label (Rect (Screen.width/2-50, Screen.height/2-25, 100, 50), "BLAH", centeredStyle);
}

Or you can make your own style, set it up in the inspector to include centering, and use that:

var myStyle : GUIStyle;

function OnGUI () {
    GUI.Label (Rect (Screen.width/2-50, Screen.height/2-25, 100, 50), "BLAH", myStyle);
}

hey there again! here's the code to do that:

    var windowWidth : int = 200;
    var windowHeight : int = 200;
    /*var here from your other script should be Str. doesn't have to be in this script though. that's what ScriptName.StaticVarName does. 
goes through that script to find that static var. 
only works for static vars as far as i know.
    */

    function OnGUI()
    {

     GUI.Window(0, Rect((Screen.width / 2) - (windowWidth / 2), 
                                   (Screen.height / 2) - (windowHeight / 2), 
                                   windowWidth, 
                                   windowHeight), Message.Str); // should be centered. 
    }

and if it's not centered, play around with the window height / width, and if it's still not working, let me know.