[denby]

Originally Posted by denby

Nice mod but it's operating in the grey zone and until Google definitively clarifies whether it's OK for a spider to see my forum content while an unlogged-in visitor can not, I will not be installing it. I can't afford the risk of losing my significant monthly GoogleAds income or my healthy SE ranking.

I will write to Google and report here if I get a clearer answer than trilljester received.

Well, they will not commit to any further elaboration so I say act on the side of caution and beware.

I have settled on a compromise: I will allow all guests (including spiders) to view the forums and thread titles but stop short of showing thread content. Hopefully most thread titles will be informative (and not of the "Help!", "One more thing" and "Well what do you know..." variety) and so entice Google search engine users to come and register.

You can achieve this with the built-in permissions options by setting the forum permissions for each forum for the Unregistered/Not Logged In usergroup as follows:
Can View Forum Yes
Can View Thread Content No
Can View Others' Threads Yes

vB can't tell who the guest is (whether or not they have posted in the forum before) and so all threads are by someone else. But they are still not allowed to view the content.

The quickest way to implement this for all your forums is to do it once manually for one forum and then use the Forum-Based Permission Duplicator . You can find a link to that duplicator at the top of the page:
Admin Control Panel -> Forums and Moderators -> Forum Permissions

I hope this helps.