title: PerformanceProgressBar "Invalid cross-thread access" exception
body: <p>I am developing WP7 app. I met some unexpected behavior. I use PerformanceProgressBar from the SilverLight toolktit in my app in several pages. These PerformanceProgressBars are binded to ViewModel property called IsBusy. Each page has its own ViewModel.</p>
  
  <p><code>....&lt;toolkit:PerformanceProgressBar 
              VerticalAlignment="Top"
              HorizontalAlignment="Left"
              IsIndeterminate="{Binding IsBusy}"
              Visibility="{Binding IsBusy, Converter={StaticResource BoolToVisibilityConverter}}"<br>
              /&gt;......</code></p>
  
  <pre><code>    public bool IsBusy
      {
          get
          {
              return this._isBusy;
          }
          set
          {
              if (value == this._isBusy)
              {
                  return;
              }
  
              this._isBusy = value;
              RaisePropertyChanged("IsBusy");
          }
      }
  </code></pre>
  
  <p>When I change IsBusy value, I get "Invalid cross-thread access"  exception.</p>
  
  <p>Any ideas?</p>