When transitioning to exchange 07 from exchange 03 (with Office 07 already deployed), is there a way to make room resource mailboxes on the 07 mbox serveravailable to users with mailboxes on exchange 03?

The room resources should show up in the GAL. Older clients should be able to see them and send schedule requests to them. I have not tried it, but I can't think of a reason why it would not work. (Of course, that being said, there is probably some sneaky reason why it would not work.)