phpBB can also be hacked easier than you want to know. The other day I watched a script kiddie take down a forum I'm familiar with (and that thousands love to death). He used a simple vulnerability (though the code isn't there anymore) which involved modifying your login cookie to make you admin. It was sad to see it happen and to watch a kid romp through a community.

Apparently phpBB is highly targeted because it's highly popular. So if you use it, make sure to stay up on patches and vulnerabilities.