You may have picked this up already but if not, it’s time to think about phpBB and security again. A couple of relevant links – Bot Authors Targeting phpBB Forums and phpBB mass hack being prepared?.
Without wishing to say anything bad about phpBB, would strong recommend investigating alternatives. Two of note – Fud Forum who’s main developer is Ilia Alshanetsky, author of this PHP security book and PunBB, which is noteworthy given that these guys use it. Both have tools to help migrate from phpBB.