Hi,
ContextMenu relies on Windowless mode, Safari sadly does not implement the Windowless mode for the Silverlight plug-in, Google Chrome which is also based on WebKit does implement it and it works fine.
In order to support Safari you need to decide how to trigger the ContextMenu, some common options are Apple + left-click or Shift + left-click. You can set the ContextMenu to DetectRightClick="False" and ContextType="Custom" then use the ShowContextMenu(point) to display it.
Currently this functionality is not built-in to the control but is fairly easy to implement.
Jeremy