It's Simple:Press, which is basically a way to make WordPress behave like a forum - you're one of so few people here who actually like it that we're changing to phpBB next week!
Yay phpBB, but security nightmare if you aren't hot on updates.
I'm not sure which phpBB version you are speaking of, the current stable line (3.0.x) has gone through a full security audit during it's RC stages and hasn't seen any serious issues since it's release. As a matter of fact, I can't recall seeing one hacked using an phpBB exploit (sure there have been compromised boards, but the POI usually is the server itself or an other (third party) pice of software).
Overall if you do some research you'll find that the phpBB 3.0.x line has a var better security report than comparable BB solutions, so "security nightmare" might be a bit of an exaggeration
Every popular CMS is prone to exploits. http://goo.gl/nMbYA
Thats why one should either write his own code or use a solution and update the moment an update arrives. I call this computer hygiene. But even then you are never 100% secured against the creativity of a resourceful attacker.
I would choose to use a Google Groups Forum to be relieved from lot's of responsibilities and yet have a extremely powerful administrable forum with nearly unlimmited resources. But PHPBB is as good as any with the downside off added responsibility for users personal data.