Ok, eu mesmo vim com a solução. Usei uma subconsulta dependente para resolver. Isto é o que tenho no resultado:
SELECT
item_discuss_threads.item_discuss_thread_id
, item_discuss_threads.item_discuss_thread_datetime
, item_discuss_threads.item_discuss_thread_title
, latest_posts.item_discuss_post_title
, latest_posts.item_discuss_post_datetime
, COUNT(item_discuss_posts.item_discuss_post_id) AS nb_posts
FROM
item_discuss_threads
INNER JOIN item_discuss_posts
ON item_discuss_threads.item_discuss_thread_id=item_discuss_posts.item_discuss_thread_id
INNER JOIN item_discuss_posts AS latest_posts
ON latest_posts.item_discuss_thread_id=item_discuss_threads.item_discuss_thread_id
WHERE
(
SELECT
item_discuss_post_id
FROM
item_discuss_posts AS p
WHERE
p.item_discuss_thread_id=item_discuss_posts.item_discuss_thread_id
ORDER BY
item_discuss_post_datetime DESC
LIMIT
1
)=latest_posts.item_discuss_post_id
GROUP BY
item_discuss_threads.item_discuss_thread_id
ORDER BY
latest_posts.item_discuss_post_datetime DESC
