public class TouchBehaviors: DependencyObject
{
  public static DependencyProperty TouchDownProperty = DependencyProperty.Register(
    "TouchDown", typeof(ICommand), typeof(TouchBehaviors),
    new PropertyMetadata(null, new PropertyChangedCallback(OnTouchDownChanged)));

  public static ICommand GetTouchDown(DependencyObject d)
  {
     return (ICommand)d.GetValue(TouchDownProperty);
  }

  public static void SetTouchDown(DependencyObject d, ICommand command)
  {
    d.SetValue(TouchDownProperty, command);
  }

  private static void OnTouchDownChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
  {
    UIElement element = d as UIElement;
    if (element != null)
    {
      element.TouchDown += OnTouchDown;
    }
  }

  private static void OnTouchDown(object sender, TouchEventArgs e)
  {
    DependencyObject d = sender as DependencyObject;
    if (d != null)
    {
      ICommand command = GetTouchDown(d);
      if (command != null)
      {
        if (command.CanExecute(null))
        {
          command.Execute(null);
        }
      }
    }
  }
} 

<Grid local:TouchBehaviors.TouchDown="{Binding TouchDownCommand}"/>