[Ipstenu (Mika Epstein)]

That is the right thing to edit! One way to do it is to add a counter

<?php

$postorder = 0;

foreach ($posts as $bb_post) : $del_class = post_del_class(); ?>

<?php $postorder++;

<li id="post-<?php post_id(); ?>" class="entry-<?php echo $postorder; ?>"<?php alt_class('post', $del_class); ?>>

<?php bb_post_template(); ?>

</li>

<?php endforeach; ?>

Then you can just use CSS and format li.entry-1 however you like!