I know there are some efforts to integrate forums with WordPress. [...] The main problem with external forum systems (as opposed to solutions purely based on plugins and themes) is that if you want a consistent look-and-feel between your blog and your forums, you have to do a lot of work.
WordPress, endlessly themeable, can be made to look like a forum, but that's not what he's talking about here. He wants feedback from other developers out there on the approaches they're taking to get WordPress to cooperate this way. There's over thirty comments on the post now with some great feedback from all around the community - check them out!