Resolved: The JBoss SAAJ implementation has some issues, the following addition in the spring-ws application context resolves it:
<bean id="messageFactory">
<property name="messageFactory">
<!-- This is the Java 6 variant of this fix! Note the "internal" package missing in the spring-ws FAQ. -->
<bean class="com.sun.xml.internal.messaging.saaj.soap.ver1_1.SOAPMessageFactory1_1Impl"/>
</property>
</bean>
See http://static.springsource.org/spring-ws/sites/1.5/faq.html#saaj-jboss for the hint. But you bet that was hard to find ;-)
hope this helps
Stefan