I'm facing the same ( or a similar ) problem.. I am trying to generate a SOAP message like:

<RetrieveServiceContent xmlns="urn:vim25"> 
<_this> 
type = "ServiceInstance" 
ServiceInstance 
</_this>
</RetrieveServiceContent>
[download]

but no matter how hard I try, I cannot get SOAP::WSDL to generate this, all it generates is

<RetrieveServiceContent xmlns="urn:vim25"> 
<_this> 
ServiceInstance 
</_this>
</RetrieveServiceContent>
[download]

The ServiceInstance is of type which has an attribute of type, but even if I set that attribute to the right value, it doesn't show up in the SOAP message..