I think this would help with site organization in general.
Consider a common topic such as vacation flags. There must be a dozen threads on this one topic.
I usually look to make sure a similar thread does not exist before creating a new one. This feature would make this easier.
Even better, reorganize the forums hierarchically by topic, much like newsgroups (e.g. http://www.cyberfiber.com/compnoto.htm) or the folders view of Microsoft Windows Explorer.