I am having exactly the same problem; works just fine when using a Java Client but when using a VB client (SoapClient30) I get a "Cannot find child element" error. Calls with no parameters work fine. Calls with any parameter, regardless of the type cause the "cannot find child element" error. I upgraded to the latest version of JBossWS (1.0.4) and it still does not work. I tried different SOAP Binding Styles: RPC/literal, DOCUMENT/literal.
Did you ever find a resolution?