Tente este (deixe-me saber se funciona):
SELECT forum_categories.title, forum_messages.author,
forum_messages.date AS last_message
FROM forum_categories
JOIN forum_topics ON forum_topics.category_id=forum_categories.id
JOIN forum_messages ON forum_messages.topic_id=forum_topics.id
JOIN (SELECT MAX(m.date) as date, top.category_id
FROM forum_messages m
JOIN forum_topics top ON m.topic_id = top.id
GROUP BY top.category_id) as t
ON t.category_id = forum_topics.category_id AND t.date = forum_messages.date
WHERE forum_categories.id=6
GROUP BY forum_categories.id
Aqui está uma segunda opção:
SELECT forum_categories.title, forum_messages.author,
forum_messages.date AS last_message
FROM forum_categories
JOIN forum_topics ON forum_topics.category_id=forum_categories.id
JOIN forum_messages ON forum_messages.topic_id=forum_topics.id
WHERE forum_categories.id=6
AND forum_messages.date = (SELECT MAX(date)
FROM forum_messages t
WHERE t.topic_id = forum_topics.id)
GROUP BY forum_categories.id
ORDER BY forum_categories.date ASC
