I also noticed that thw WDSL definition does not contain a description of the content of the Response, it simply states "any"
Might this be the problem?
If you mean xsd:any, then that's the trouble (you might include that part of WSDL file - it could help).
See what problems you might encounter with xsd:any:
http://www.ibm.com/developerworks/xml/library/ws-tip-xsdcaution.html
xsd:anyType, an alternative to xsd:any, is another story. I have successfully used xsd:anyType with rpc/encoded many times (but within PHP only!).
PHP SoapClient/Server is able to send e.g. dynamic associative arrays in such configuration without losing index names, which is very helpful sometimes.
If you plan to make your web service available to different clients (JAX-WS, Axis, .NET), eliminate the need for 'any' elements - it is much better (if it is possible) to write full type definition of sent data.