Mine

mysql> EXPLAIN SELECT root.messageid, IFNULL(( SELECT MAX(reply.senttime) FROM zeroseventwo_forummessages reply WHERE reply.thread = 2 AND reply.touid = root.messageid), root.senttime) as lasttime, root.thread FROM zeroseventwo_forummessages root WHERE root.thread = 1 ORDER BY lasttime DESC;
+----+--------------------+-------+------+---------------+--------+---------+-----------------------+------+-----------------------------+
| id | select_type        | table | type | possible_keys | key    | key_len | ref                   | rows | Extra                       |
+----+--------------------+-------+------+---------------+--------+---------+-----------------------+------+-----------------------------+
|  1 | PRIMARY            | root  | ref  | thread        | thread | 2       | const                 |   80 | Using where; Using filesort |
|  2 | DEPENDENT SUBQUERY | reply | ref  | thread,touid  | touid  | 5       | phpdip.root.messageid |    4 | Using where                 |
+----+--------------------+-------+------+---------------+--------+---------+-----------------------+------+-----------------------------+
2 rows in set (0.00 sec)