I agree with jeff666. I've run a phpBB system for a couple years. Its popularity attracts all sorts of hackers and spammers. By using an IP tracking feature that I installed I can follow how they found my place. They do it by just inputting simple common phpBB index-page strings like "po*ered by phpbb" into google, then they send out their bots to hit all phpBB boards they can find. This is still going on daily, relentlessly. (You should see the extensive, and I do mean extensive, domain ban list that I've had to asssemble over 2 years. Spammers from some countries are so intensive that I had to ban whole countries.) I also had to revamp the whole sign-up routine on mine just so their bots couldn't match the default fields, and yet some still get through on a weekly basis. (Probably manual spammers signing up by hand.)
And, while the phpBB system is extensive for MODS and tweaks and interesting things you can add (to say the least, my own forum has over 50 custom features that I added and then tweaked to make them even more neat), the new phpBB has ONE theme for it, not a very attractive one either. It reminds me of a clunky and kid-like play-dough container for some reason. All old MODS for phpBB 2.x won't work with 3.x yet. So, give it a few years for all the MODers to revamp their code. Plus many of them have moved onto other things and none of the way cool features from the past won't even be ported to the new 3.0.
I say, keep it as it is. Using that white and blue theme here is really attractive, and the board does all it really needs to do. As the ol' saying goes, "if it ain't broke DON'T FIX IT!" :-)
p.s. acseven, after you have read this, it might be good to edit that "powered by ..." string in this post. That alone showing up in google is enough to send them this way.