Hmm...Andrew, good name. 😉
This is one of those things that can either be very painful to do, or an easy hack (but ugly). You should look through the login scripts for the PHPBB forum as closely as possible. That way you can get a good idea of how they do things.
From there, you should really just setup a login script to emulate their system. You'll need to query their user db, match usernames/passwords, and then create the same cookie/session variable that they use.
Keep in mind, this is just how I'd do it personally, but it's a start, and probably a good learning experience for you. If you need some help with it, you know where to ask.
