I am not sure why the property is not being called on Binding. Here is the code:

<myusercontrol
Text ="{Binding Description, UpdateSourceTrigger=LostFocus,Mode=TwoWay, ValidatesOnDataErrors=True}" 
 IsReadOnly ="{Binding AllowEditing}"
/>

And here is the myusercontrol IsReadOnly property:

 public static DependencyProperty IsReadOnlyProperty = DependencyProperty.Register("IsReadOnly", typeof (bool),
                                                                                          typeof (
                                                                                              myusercontrol));


        public bool IsReadOnly
        {
            get
            {
                return ((bool) GetValue(IsReadOnlyProperty));
            }

            set
            {
                MessageBox.Show(value.ToString()); 
                SetValue(IsReadOnlyProperty, !value); 
                OnPropertyChanged("IsReadOnly");
            }
        }

The message box is never displayed! Any ideas!