There is a similar integration with punBB.
Ian Huston wrote a plugin which is working for posting and commenting between blog and forum.
Details:
Author’s blog and forum:
http://www.ianhuston.net
punBB forum topic on plugin:
http://forums.punbb.org/viewtopic.php?id=8373
Style integration:
http://www.liewcf.com/blog/archives/2005/04/integrate-punbb-forum-into-wordpress
(There is one more similar description somewhere)
You may download this plugin from my domain:
http://love-unlimited.net/test/wp2punbb
Use this download, the original will not work due to a minor bug.
You need to install punBB, fill out new option page in wp-admin panel and insert into you theme’s index.php the followin line:
<?php ih_add_comment($post->ID); ?>
where you want it, possibly next to or instead of the original comment tag.
If you use accented characters you need to change the charset definitions in the tpl files from:
<meta http-equiv="Content-Type" content="text/html; charset=<pun_char_encoding>" />
to
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
Now, you either use it on its own, insert it into a page in an iframe like here:
http://love-unlimited.net/workshop/?page_id=4
or you jump into liewcf’s customisation:
http://www.liewcf.com/blog/archives/2005/04/integrate-punbb-forum-into-wordpress
Well, there are limitations, like you should use one dedicated forum for cross-posting and commenting, html tags will not work because punBB uses bbcode though there is a suggestion to correct it at the end of the pubBB forum topic (see above), etc but overall it works.
I hope, I did not miss anything important.
