CREATE TABLE IF NOT EXISTS `pm` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `user_id` int(10) DEFAULT NULL,
  `useru_id` int(10) NOT NULL,
  `date` int(10) unsigned NOT NULL,
  `body` text NOT NULL,
  PRIMARY KEY (`id`),
  KEY `user_id` (`user_id`),
  KEY `useru_id` (`useru_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

SELECT p.user_id, p.date, u.id
FROM (
    SELECT IF(useru_id=2,user_id,useru_id) as user_id, MAX(date) as date
    FROM pm
    WHERE useru_id = 2 OR user_id = 2
    GROUP BY IF(useru_id=2,user_id,useru_id)
) as p
LEFT JOIN users u ON u.id=p.user_id
ORDER BY p.date DESC