[kh99]

You could do something like this:

SELECT  forum.title_clean, thread.title, post.title, post.username, post.dateline, post.pagetext
INTO OUTFILE 'posts.csv' FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' LINES TERMINATED BY '\n' 
FROM post
    LEFT JOIN thread ON (post.threadid = thread.threadid)
    LEFT JOIN forum ON (thread.forumid = forum.forumid)
WHERE thread.visible = 1 AND post.visible = 1

I imagine you'll need to tweak some things, but this might be a starting point.