this is the solution to wrap the content after the title like this
'before_widget' => '<li>',
'after_widget' => '</div></li>',
'before_title' => '<h2 class="widgettitle">',
'after_title' => '</h2><div class="widgetcontent">' ); ?>
the important markup is the <div class="widgetcontent>"
in the after_title, and the closing </div>
in the after_widget, this will result in this html markup:
<ul>
<li>
<h2 class="widgettitle">Title</h2>
<div class="widgetcontent">
<!-- widget stuff-->
</div>
</li>
</ul>