I never lock the threads on my forums.

However, from a user point of view, it probably depends on the type of forum - is it a current affairs time sensitive forum, or a tech help archive for example?

From a search engine point of view, perhaps it may have the advantage of - once the thread has established itself as being about a specific subject - that this subject does not then become 'diluted' later down the line if the discussion takes a different route.

For example, if the thread is about "Finding Blue Widgets in Spain", the discussion may contain lots of useful advice and keyword rich discussion about the "Spanish Blue Widget Industry". If the topic lies 'fallow' for a few weeks and is then picked up again, it may be that the discussion takes a detour into other territory, such as Widgets in other countries and perhaps even - shock horror - red widgets.

Of course locking an older thread also requires users to start new topics on the subject, with different titles - perhaps this time about Red Widgets. Which is also good.

Just my 2C though. I'd be fascinated to hear other people's thoughts.

The biggest problem I see is that odd old threads get reactivated rather than starting a fresh thread with current info. Of course, sometimes the old thread is highly relevant to the new info and it would make a lot of sense to continue the discussion.

A less important issue is that some percentage of users are oblivious to post dates, and will say things like, "I'm going to that conference, too, hope we can meet" without realizing that the previous post was two years ago. :)

I don't lock threads, but I don't bump threads either.