I have the following class (abreviated for simplicity). The app it multi-threaded so the Set and Get are a bit more complicated but should be ok.

namespace News.RSS
{
    public class FeedEngine : DependencyObject
    {

         public static readonly DependencyProperty _processing = DependencyProperty.Register("Processing", typeof(bool), typeof(FeedEngine), new FrameworkPropertyMetadata(true, FrameworkPropertyMetadataOptions.AffectsRender));
        public bool Processing
        {
            get
            {
                return (bool)this.Dispatcher.Invoke(
                     DispatcherPriority.Normal, (DispatcherOperationCallback)delegate { return GetValue(_processing); }, Processing);
            }
            set
            {
                this.Dispatcher.BeginInvoke(DispatcherPriority.Normal,
                    (SendOrPostCallback)delegate { SetValue(_processing, value); },
                    value);
            }

     }

    public void Poll()
    {
        while (Running)
        {
            Processing = true;
            //Do my work to read the data feed from remote source
            Processing = false;
            Thread.Sleep(PollRate);
        }
        //
    }
}

}

Next I have my main form as the following:

<Window x:Class="News.Main"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:converter="clr-namespace:News.Converters"
    xmlns:local="clr-namespace:News.Lookup"
    xmlns:rss="clr-namespace:News.RSS"
    Title="News" Height="521" Width="927" Initialized="Window_Initialized" Closing="Window_Closing"  >
    <Window.Resources>
        <ResourceDictionary>
        <converter:BooleanConverter x:Key="boolConverter" />
        <converter:ArithmeticConverter x:Key="arithConverter" />
           ...
        </ResourceDictionary>
    </Window.Resources>
    <DockPanel Name="dockPanel1"  SnapsToDevicePixels="False" >
        <ToolBarPanel Height="37" Name="toolBarPanel"  Orientation="Horizontal" DockPanel.Dock="Top" >
            <ToolBarPanel.Children>
                    <Button DataContext="{DynamicResource FeedEngine}" HorizontalAlignment="Right" Name="btnSearch" ToolTip="Search" Click="btnSearch_Click" IsEnabled="{Binding Path=Processing, Converter={StaticResource boolConverter}}">
                    <Image Width="32" Height="32"  Name="imgSearch"  Source="{Resx ResxName=News.Properties.Resources, Key=Search}" />
                </Button>
                ...
    </DockPanel>
</Window>

As you can see I set the DataContext to FeedEngine and Bind IsEnabled to Processing. I have also tested the boolConverter separately and it functions (just applies ! (Not) to a bool).

Here is my Main window code behind in case it helps to debug.

namespace News
{
    /// <summary>
    /// Interaction logic for Main.xaml
    /// </summary>
    public partial class Main : Window
    {
        public FeedEngine _engine;
        List<NewsItemControl> _newsItems = new List<NewsItemControl>();
        Thread _pollingThread;

        public Main()
        {
            InitializeComponent();
            this.Show();
        }


        private void Window_Initialized(object sender, EventArgs e)
        {
            // Load current Feed data.
            _engine = new FeedEngine();
            ThreadStart start = new ThreadStart(_engine.Poll);
            _pollingThread = new Thread(start);
            _pollingThread.Start();
        }

    }
}

Hope someone can see where I missed a step.

Thanks.