gistfile1.cs

for (int i = 0; i < yourListBox.Items.Count; i++)
{
	ListBoxItem item = (ListBoxItem)yourListBox.ItemContainerGenerator.ContainerFromIndex(i);
	if (item != null)
	{
		VisualStateManager.GoToState(item, "BeforeLoaded", false);
	}
}

yourListBox.Opacity = 1.0;