NotAuthenticated fault when dealing with 6.0 system versus 5.5 using UserSession sessionCookie

I am running into a problem when accessing the MOB for a 6.0 vCenter server setup versus a 5.5 setup.  When I try to get the attributes for a host from the MOB (my backend Java code through the VIM25 library) I get back NotAuthenticated Faults instead of the property values.  This same code runs fine against a 5.x setup.  


I pass to my backend the sessionCookie that I retrieved from the User Session (i.e. get the UserSession from the UserSessionService, get the serversInfo, and then get the sessionCookie from the ServerInfo).  


On the backend side, I setup the vmware_soap_session cookie string with the session cookie that I passed over.  I put the cookie in the request context HTTP_REQUEST_HEADERS and use this to get the service content from the VIM port.


I then get the property collector from the service content, setup my property filter spec and then fetch the properties.  Against a 5.5 server I get back the properties correctly, but against a 6.6 system I get the fault.