Well, here's what I did, but I don't want to doc it fully as I don't want to support it - if you catch my drift. Just making them list is easy (and I'll describe), making them show new posts and post counts is a bit more involved...
1) First, you just create the forum - making sure it appears below the forum you plan to make it a sub of.
2) In ubbthreads.php, you place this line just above where the $forum variable gets assigned.
if ( $Keyword == "yoursubforumkeyword" ) continue;
This will prevent the subforum from being displayed on the Main Index.
3) In the description field for the Parent, you insert a link to the sub-forum:
Description text<br /><b>Sub-Forums:</b> <a href="/postlist.php...">Sub-Forum Name</a>
So, you actually create a hard link in the description to the sub forum.
That's it! You have a sub-forum now.
That's how I did the Main Index. I also added some code to explicitly add a "Sub-Forum:" and "Parent:" link when viewing those two forums to the postlist.php script. (blank is in other forums).
Also, on the ubbthreads.php script I modified the $forum[$x][$y]['Description'] variable for the parent to append a NEW tag if there are new messages (which we already know because its been checked for).
if ( $Keyword == "subforumname" ) {
$olddesc = $forum[$otx][$oty]['Description'];
$newdesc = str_replace( "</a>", "</a> <img border="0" height="12" width="16" src="/images/newposts.gif">", $olddesc);
$forum[$otx][$oty]['Description'] = $newdesc;
}
And, finally, the trickiest part, I added code to check for new messages in the postlist.php script *if* I was viewing the parent.
Thanks about it. Like I said, this was specific to my forum and your milage may vary.
I will probable try and tackle this after I move in a month but if ya do let me know 
