Hi everyone. I'm back from vacation, and I'll try to answer some of your questions.
FrancWest, I'll ask the DAV developers if this is possible. If possible, I strongly suggest that you separate the CAS role from the MBX role.
Evan, I'm sorry you've had a rought time with Exchange 2007. Can you detail some of the problems you're encountering? They may be scenarios we haven't thought about, or ones that we're addressing in an SP or in E14.
Toshio, for performance reasons, OWA always tries to direct you to a CAS in the same AD site as the mailbox you are accessing. In your case, it seems you want to use proxy instead of HTTP redirection. proxy requires that the OWA vdir has both InternalUrl set and Windows Authentication turned on. If both sites are exposed to the internet, redirection may be a better option.