Unity3D GUI Resolution

gistfile1.js

function OnGUI() {
     var screenScale: float = Screen.width / 480.0;
     var scaledMatrix: Matrix4x4 = Matrix4x4.identity.Scale(Vector3(screenScale,screenScale,screenScale));
     GUI.matrix = scaledMatrix;

     // then do the rest of your GUI as per normal, using the 480x320 screen size you had for your standard res iPhone app
}