Can Feature layer be identified?
But my mapservice layers and feature service layers failed to identify.
No, the identify task is not working with feature layers. As the features are loaded at the client side, you don't need to indentify the features ('Identify' is working with the server). Instead you can use a maptip to display infos about features.
It should work with mapservice layers. What is exactly your issue?
To test if IdentifyTask failed in your SL application or if empty IdentifyResults was expected, you can use Fidder to see what parameters SL app formed and use the same parameters in your web browser. If the result that you get from the web browser is not identical to the result you get from your application, we may have a problem. To do this, you can look at this thread, post #14 http://forums.arcgis.com/threads/14730-Area-And-Perimeter.
ESRI.ArcGIS.Client.Tasks.IdentifyParameters identifyParams = new IdentifyParameters()
{
Geometry = clickPoint,
MapExtent = map.Extent,
Width = (int)map.ActualWidth,
Height = (int)map.ActualHeight,
LayerOption = LayerOption.visible,
SpatialReference = map.SpatialReference
};