Yes, the latest posts. The main category should contain posts from all, or specified forums. I was trying to use this, but it wouldn't work:
// create sql statement
$sql = "SELECT threadid, title, forumid, replycount, postusername, postuserid, lastposter, dateline, iconid FROM thread WHERE forumid = \"2\" AND forumid = \"3\" AND forumid = \"4\" ORDER BY threadid DESC LIMIT 5";
Additionally, each fourm should be it's own category, with the latest posts from that forum only (I hope that makes sense

). The author said categories would be easy to implement, but no one has created them yet.