Mergeforums.php - anyone else doing this?
 

City-data.com uses a php script to merge subforums into main forums, and display them as one.

An example can be viewed here:
http://www.city-data.com/forum/mergeforums.php?f=20

Do you know anyone else doing similar, or a mod that implements something like this?

I can think of a way to do it via search.php, but the functionality isn't exactly the same.

According to the url the php file is mergeforums.php, so that's probably a custom script. I imagine that you could make a copy of forumdisplay.php and modify the queries to include threads from child forums. It doesn't look to me like it can be done via plugins.