This has probably already been discussed, and I don't know how beneficial it would be anyway, but....
would it be possible to have an 'archive' feature which the forum admin could run whenever, that would take out messages/threads that have been inactive for a given length of time and put them into a flat HTML file to save space/load/complexity on the database?
You [by which I mean the beehive devs (by which I mean you :$ )] could set the database to redirect the message numbers to the flat-file with # tags or whatever, and it'd probably be relatively simple to preserve the ability to reply directly to the individual messages too.