create a new user field called postsorder and set it to default @ 0 (oldest first) and do all the stuff in member.php to allow the user to change it, etc.
in showthread.php find
Code:
// oldest first or newest first
if ($postsorder==0) {
$postorder="";
} else {
$postorder="DESC";
}
and change it to
Code:
if ($userinfo[postsorder]==0) {
$postorder="";
} else {
$postorder="DESC";
}
course i've never tested that, but it should work nicely