No. The ArcGIS Silverlight API is designed to work with ArcGIS Server services via REST, which was first introduced with ArcGIS Server 9.3. To use SOAP services (ArcGIS Server and otherwise) in Silverlight, you need to generate the client side proxies. The .NET SDK and Silverlight SDK include a SOAP toolkit to generate the proxies for you. New in Silverlight 3 (and available in 4) is the SlSvcUtil command line utility to generate SOAP proxies for the Silverlight platform. You'll find the utility in %Program Files%\Microsoft SDKs\Silverlight\v4.0\Tools. You can download the wsdls for ArcGIS Server 10 services here: http://help.arcgis.com/EN/arcgisserver/10.0/apis/soap/Pre-generated_proxies.htm
Once you have the wsdls, you can use SlSvcUtil.exe to generate a single class that contains all the proxies and value object types:
Note the parameter "*.wsdl" which indicates that all wsdl files in the current directory will be processed. You will see a series of errors indicating that numerous types have already been declared. This is expected since the same types are defined in multiple wsdls - you can ignore the error.
