I have a construction like above, we just auto archive everything older then "x" days in certain forums.
Instead of manually selecting what to archive, we can manually select topics to keep, so that they stay in the main forum.
Archive forum is even on a different server in our case.
All in all, it's a few db+ubb changes (fields and code to keep/unkeep the topics), and one extra script that exports the topic data as mysql queries.
Sort of "archive.php" that as a result gives a mysql dump of one complete topic.
The receiving end needs to do cronjobs to receive the data, and run those queries. And afterwards call a fixup scripts that updates some tables in ubb with the new posts information.
I am more then willing to share the code, but I won't do this public here yet because "as-is" you need to be a bit of a techy. PM me if you want it.
Owh, and because our forum doesn't use attachments in any user forums, my archiving stuff does not take those in account.
Last edited by blaaskaak; 03/12/2009 5:28 PM.