You'll need to get around Magento's bootstrap (/index.php
), because phpBB won't function if Magento is intercepting every URL.
There's also the issue of Magento and phpBB not sharing authentication systems, so users have to log into one, neither, or both. That's not a great UX if you ask me, but solving that problem is a very complicated issue.
Other than that you shouldn't have many problems.