LayoutUpdated is the last object lifetime event to occur in the sequence prior to a control being ready for interaction. However, LayoutUpdated also can occur at run time during the object lifetime, for a variety of reasons: a property change, a window resizing, or an explicit request (UpdateLayout or ApplyTemplate).
In WPF, this event is a routed event. In Silverlight, this event is not a routed event; it is a standard CLR event that uses EventArgs, not RoutedEventArgs.
LayoutUpdated can occur in cases where the object that raises the event does not necessarily change the visual tree under it. For instance, imagine a layout container where there are two elements. If the first object changes a property that forces a new layout, then both objects raise LayoutUpdated because the second object might be repositioned even if its own subsidiary layout does not change.