I think stopping unregistered people from viewing your forum is a huge mistake.
Its nice for invite-only places, but really if you're counting on walk-ins, what's to make anyone bother? your domain name? You're just one of several million websites... Most people arent going to hang around long enough to even think about registering, unless there's something there of interest...
... at least thats my reasoning... but honestly, who knows what goes through the minds of AIM users