I wonder how I missed these great forum software before; MercuryBoard and QuickSilver. They tend to feel alot like phpbb, but from what they say are completely rewritten which means there is a good chance the code is much cleaner than in phpbb, and more standards compliant.
In fact, QuickSilver generates clean and compliant XHTML 1.1 code on both index page and topic view pages, which is I must say quite impressive for any dynamic CMS such as forums. I am aware that it depends alot on the theme used, but in some cases the CMS, such as phpbb, forces the use of noncompliant stuff.
Another great improvement of mercury board and quicksilver (which is based on mercury) is that it goes a step further in modding. Finally you are able to just download the mod file and put it in a "modlets" directory. After that you only need to execute some SQL commands using the provided admin panel (if sql is even needed for the given mod) and that's it. I imagine something like that would be so much easier to maintain.
Anyway, it's quite intriguing, and gives me hope that there is actually a good, standards compliant and modern alternative to phpbb out there, that is also fully featured.
You can check the forums out by testing it in their (oddly named) spam forum.
You're welcome to post what you think of it.