With UserException, as defined by

Code:

<complexType name="UserException">
<sequence/>
</complexType>

you are indeed right, and a message element should not appear on the wire.

Code:

<soapenv:Fault>
<faultcode>soapenv:Client</faultcode>
<faultstring>org.jboss.test.webservice.exception.UserException</faultstring>
<detail>
<ns1:UserException xmlns:ns1="http://org.jboss.webservice/exception/types">
<message xsi:nil="1"/>
</ns1:UserException>
</detail>
</soapenv:Fault>