Jo klar, so müsste mans dann glaub machen: (statt lastpostdate natürlich mit lastpostid)
SELECT comments.board, comments.thread_id, max( comments.lastpost ) AS lastpostdate, lastpost.id AS lastpostid, lastpost.text
FROM `comments`
LEFT JOIN comments AS lastpost ON ( lastpost.date = comments.lastpost )
GROUP BY comments.thread_id
ORDER BY lastpostdate DESC
LIMIT 0 , 30
