Interesting idea... I've done it before, it's not a hideous amount of work, just requires a careful fudge of a few things.
Basically, when you're inside a board or topic, you get the option to quickly search 'this board'. It could be nice to offer 'this board and all sub boards' as one of the dropdown options.
The trick is to grab the option when doing the preliminary search checking (when it validates the content of search_selection as well as sd_brd[]) and to do a quick search to get all descendants at that point, and add them to the list of boards you're searching, just as if you'd selected search with a list of boards normally.
It sonuds worse than it is, actually...
Yep, that's a very nice and interesting idea!
It's a while I randomly think about it (i.e. when I need it I think "darn it would be cool to have that", then I forgot and continue doing what I was doing before... lol), but never had the opportunity to even check what that would involve...
It's really not a big amount of code to do, I implemented it in a single night in Wedge ;)