Very nice! Can you also give us your website so people can check out the usage of the mod? 
I will of course

I'm developing it locally at the moment I will upload it to our web servers later on the month. Right if your interested this is the solution to the collapsible problem
Step 1: Open "yourforumdir/Sources/BoardIndex.php" and find the following code
$result_boards = db_query (around line 71)
Step 2: Add above this the following code (might look familiar)
// Added for the multiple forum mod
if(!empty($_REQUEST['forum']) && !isset($_REQUEST['board']) &&!isset($_REQUEST['topic'])){
$forumList = db_query("SELECT catList FROM {$db_prefix}forums WHERE forumName = '$_REQUEST[forum]'", __FILE__, __LINE__);
$row_forumList = mysql_fetch_assoc($forumList);
$user_info['query_see_board'] .= ' AND FIND_IN_SET(c.ID_CAT, "' . $row_forumList['catList'] . '")';
}
Thats it as far as I know.