This probably exists - show guests only thread titles

Did a search in finished hacks but couldn't find it.

Basically what would be cool would be the option of only showing guests thread titles in selected forums, i.e they can click on the forum from the forum home page, they then can see the normal listing of threads, but they can't click into a thread to read what's going down.

Sorry if this already exists, but was unable to locate through the search function.

Cheers,
J.