I'm posting this on behalf on a colleague who posted this bug on the ode-users mailing list.
I've generated instance management clients from pmapi.wsdl and the wsdl supplied by the instance management service. The method getVariableInfo() of such a client always returns null. The response message is not empty, we could verify this using SoapUI and tcpmon. The same problem exists with a generated C# client, so a bug in JAX-WS / JAXB is highly unlikely. We figured there's something wrong with the response message.