[Lynne]

Try this (note parenthesis in FROM statement):

SELECT thread.threadid, thread.forumid
FROM (thread AS thread, subscribethread AS subscribethread)
LEFT JOIN deletionlog AS deletionlog ON(deletionlog.primaryid = thread.threadid AND type = 'thread')
WHERE subscribethread.threadid = thread.threadid
AND subscribethread.userid = 1
AND thread.visible = 1
AND lastpost > 1189059102
AND deletionlog.primaryid IS NULL