So, how about something like say a button to klick, to mark a thread a technical issue (to contrast against those "...drink my beer.." or "...Nokia goes Windows..." threads). A moderator would be informed by that and can then send a final post,
- linking to either an existing or a blank wiki page,
- calling users to transition the info from the thread to wiki and continue discussion on the corresponding wiki discussion page
- blocking this thread against further posts.
As a result of this, a new topic can be discussed, maybe after about 30 posts some useful hints should be collected so far, so these can be moved to wiki. Where topics are repeated in new threads (by not searching before posting) a hint pointing at the wiki page should appear within say the first 5-10 posts. So, these threads can be closed as well, by this technique.
This will not generally reduce the number of threads, but the mass of posts with redundant information, improving the overview, which, as I understand correctly, was your intention.

One precondition to make this really usable, in this I fully agree with slender, was to merge the two user bases from garage and talk.maemo.org. This is really a bit annoying.

To those, who shy away from wiki markup: Unformatted collected information is much better than no information. Reorganization and formatting can be done by someone else who is more used to wiki (it's not too hard). Wiki is not for creating the complete, perfect documentation in one shot.

Another nice feature, I saw, but I don't exactly remember where (maybe bug-tracker of launchpad or so) is the following:
The subject is checked and compared against existing ones. So on creation of a new thread, a list of existing subjects is presented accompanied by the question, if one of these subjects covers the question. Only if this is denied, the new thread is actually created.

I'm no expert in forum software, so I don't know if this is available, in any way.