If you any are thinking about starting a forum then I recommend that you take a look at the SEO Mod created by Phpbb-SEO. This featured is great when it comes to making your forum SEO friendly. We've all seen forums that have those ugly url extentions, phpbb-seo can take care of that for you.
I won't go into all of the details about the plugin all I will say is that works great and worth a try if you want an seo friendly forum. For all of the details about this plugin please visit their website they will be able to answer all of the questions you might have.
Thanks and enjoy.