MouseButtonEventArgs defines Handled, which is relevant for marking a UIElement..::.MouseLeftButtonDown or UIElement..::.MouseLeftButtonUp event as handled. Handling the event prevents typical handlers further along the route from acting on the same event occurence. For details on how to mark mouse events as handled, see Mouse Support.
Silverlight 2 does not expose right mouse-button events, because these are not relevant for some of the platforms where Silverlight can be used. Therefore the Silverlight version of MouseButtonEventArgs does not report ButtonState or a number of the other concepts supported under WPF.