using: http://codex.wordpress.org/Function_Reference/the_content#Overriding_Archive.2FSingle_Page_Behavior
and the $strip_teaser parameter:
http://codex.wordpress.org/Function_Reference/the_content#Usage
in single.php, replace <?php the_content(); ?>
with:
<?php if( strpos(get_the_content(), '<span id="more-') ) : ?>
<div class="before-more">
<?php global $more; $more=0; the_content(''); $more=1; ?>
</div>
<?php endif; ?>
<?php the_content('', true); ?>