In TinyPortal this is done like this: usually when you call index.php without anything behind the "php" it assume you want to go to the main page - the boardindex. If you type index.php?board=xx it goes to the actual board, and so on....
TP just changes that to go to index.php?action=tportal instead when you write nothing, and use index.php?action=forum to actually go the boardindex again. ( you will never see the action=tportal bit...)
So if you want this on your site then there is a couple of problems....Since your frontpage is really outside the forum, it needs to be somewhere else - since the forum NEEDS index.php, even if no longer go to the boardindex( it will do that anyhow, since it now hasn't got anything defined for "nothing" behind index.php. Just "popping" back to the default page, which in errors always are boardindex.).
As danolie describes it can be done with bringing in SSI.php on the frontpage.In your situation I would just do it like that, having the forum in a separate folder. Just add a extra button in the loon theme that says "frontpage" or something - and that points to the actual frontpage again.