I have a web service method that returns an XMLDocument.  When I call it from 
an aspx page, the intellisense says that the method is of type XMLNode, so 
when I try, stupidly, to get the results of the web service call into an 
XMLDocument in the client, it fails with the error

System.Web.Services.Protocols.SoapException: Server was unable to process 
request. ---> System.InvalidCastException: Unable to cast object of type 
'System.Xml.XmlElement' to type 'System.Xml.XmlDocument'.

Does anyone know where I might find out more about this?

Thanks