When a feature service has a Unique Value Renderer set up in ArcMap and the <all other values> is NOT checked and one value is represented you can't edit using a Silverlight API application.
One workaround the customer identified was that if you Check <all other values> you can edit. However, you can see the features in that other category. However you can symbolized them with no fill and no outline. So users can't see them, but if a user accidentially clicks on the area where on is that polygon becomes selected and can be seen in blue. And it can be deleted.One other workaround is that you could set a definition expression on the layer.