[rant]Well, piersk I disagree to an extent with your comment. I say that the reason phpBB has security "issues" is because it's so popular with the "unenlightened" crowd. The unenlightened crowd being the joe-schmoe someone that just bought a cheap web-host plan and used cPanel to install phpBB via Fantastico and run his own community forum.
If you know what you're doing, and you stay up on it, and subscribe to the phpBB mailing list, it's really as secure as anything else. The reason it's targeted is because it's free and there's the most chance for success because of novice website designers/maintainers.
Now, that being said, vBulletin is not actually completely secure either. vBulletin has its share of vulnerabilities and problems. The reason you see less is because this is a Pay-for-use application. Not too many normal people will shell out hundreds of dollars for something that isn't going to make them money, or augment that which does make them money.
And any code is only as secure as you make it. I could create some rock-solid code, or some loosey-goosey code. It's up to the coder. phpBB tries to code to the most secure standards while also being universal in PHP versions.
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Now, about the whole thing with adverts, any forum software will allow it, you just have to modify the template. And if you can't find time to modify the templates in phpBB, you most likely won't find time to modify any script to your liking.
Now, I'm not sure about all these softwares, but I do know that phpBB3 (Olympus) is coming soon, and phpBB2.1 will be history. And just like that, YaBBSE is gone, and SMF is in its place.
I finally have a use for this!!
You can compare all these boards side-by-side (phpBB2, phpBB3, SMF) at:
phpBB 2 (current stable release)
phpBB 3 (Olympus) (CVS build as of 1-22-06)
Simple Machines Forum (SMF) (current stable release)
Now, there are other forums that I'm adding, but it's difficult to get them running because they are based off of phpBB (what a surprise) and take some tweaking.
Everything takes time, so you kind of have to have it to make it work. I'd suggest just sticking with phpBB rather than converting to vBulletin or FUDForum as it will be more work for you....