Using Exchange 2010 SP1. Have a Federation Trust set up through Federated Gateway with an affliated organization. Using 3rd party GALSync product (not through Forefront) to share some address lists - My users and conference rooms get created as mail-enabled contacts in his AD, and visa versa. Both of us have receive connectors specific to the other's mail servers.
When one of my users tries to book a meeting in his conference rooms, the system responds correctly- the room books the meeting if it's in-policy, and sends the response to the organizer. When one of his tries to do the same in mine, my room books as "tentative" and doesn't reply - the way a 2003 server "resource room" used to reply if the user put the conference room in the "Attendees" field instead of in "Resources." In fact, when I'm testing this using OWA scheduling assistant (logged in as a testuser in his domain), I'll enter the address of one of my rooms in under "Select Rooms" and it immediately tells me "Attendees are not allowed in the Rooms list and have been moved to the Attendees list" - and then the conference room shows up under "Select Attendees."
We have compared configuration until our brains hurt. The Resource scheduling and policy info on each room, his and mine, are the same. All the rooms have "ProcessExternalMeetingRequests" set to True. What should we be looking at to troubleshoot this?