Silverlight TextBlock not binding to MVVM, what have I missed?
===

MainPage.xaml  
`<TextBlock Text="{Binding Pathname, Source={StaticResource ViewModel}, Mode=OneWay}" />`

App.xaml  

    <ResourceDictionary>
         <vm:InspectViewModel x:Key="ViewModel" />
    </ResourceDictionary>

ViewModel    

    private string _pathname = null;
    public string Pathname
    {
        get { return _pathname; }
        set
        {
            if (_pathname != value)
            {
                _pathname = value;
                RaisePropertyChanged("Pathname");
            }
        }
    }

    public void UpdatePathname(string path)
    {
        Pathname = path;
    }

MainPage CodeBehind  

    private void lazyNavTree_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)  
    {          
        InspectViewModel vm = new InspectViewModel();        
        var path = view.GetPath().ToArray();
        string pathname = null;
        // to figure out what the pathname is
        for (int i = 0; i < path.Count(); i++)
        {
            TreeList treeItem = (TreeList)path[i].Key;
            if (i == path.Count()-1)
                pathname = pathname + treeItem.Name;
            else
                pathname = pathname + treeItem.Name + " : ";
        }
        vm.UpdatePathname(pathname);
    }

The bound TextBlock shows nothing, nada, zip. The pathname shource is changing correctly but nothing seems to happen when I fire the INotifyPropertyChanged event on change.

I am sure I'm missing something really obvious but I can't figure out what!