Instead of FeatureLayer, use a GraphicsLayer + a QueryTask, and add the features to the graphics layer that return from the QueryTask. In your update method, you then set a where clause that specifies the added/modified date to be newer than any features you already have. Ie something along the lines of:
query.Where="LastUpdated>'"+(from a in layer.Graphics select ((DateTime)a.Attributes["LastUpdated"])).Max().ToString() + "'";