Good discussion, as always. I'll jump in late to the game with some additional thoughts.
I think primarily Edward's "Think before you post" message is the key. As long as we are conducting ourselves reasonably, we shouldn't have too much to fear.
Closing off the forums would have the detrimental effect of shutting out all those folks who find us exactly because the information they're looking for is open and public. To put some numbers behind that, in January we had 126,585 visitors as a result of search engine hits (26,399 keywords and 61,110 phrases were used to find us). Furthermore, 1,302,643 hits came to us via external links from other sites - typically cross discussions linking relevant topics here. Going private would shut these folks out. While the overwhelming majority probably come here, harvest their info and quietly leave again, I would propose that a small percentage register, stay and contribute.
I also fear that closing our doors would give people the false impression they can post without consequence. We have enough trouble policing things now without fighting the "but nobody else can see it" argument.
And lastly, for those who are compelled to have 'internal only' discussions, we do have the
Subscriber's area which is private and closed.

Cheers
Mike