Well working from the experience of a classifieds system each forum would have an id and that id either would be the child of another or it would be a stand alone forum (that may or may not have children).
eg
Australia (id 1)
NSW (id 2)
Thredbo (id 3)
An id lookup table in the database indicates that 3 is a child of 2 and that 2 is a child of 1.
Hence anything posted in 3 also appears in 2 and 1. Anything posted in 2 appears in 2 and 1 but not 3.
Each thread has the forum id attached to it so the thread knows where it lives. If a thread that lives in id 3 is viewed in forum id 1 and is replied to from there it doesn't matter the thread still lives in forum id 3 and so the reply is attached to that thread and that forum (id 3).
Doesn't seem that hard really. As I said classified systems already use it and it is a simple parent child relationship in the database.
Maybe I should put this in the suggestions forum now. Feel free to move it.
