Okay, I was looking to get this implemented on my own forums... and figured out how to do this. This works for both 2.0.1 AND 2.0.2.
First, in index.template.php (must be done for each theme folder separately), find
$settings['message_index_preview'] and set false to true.
Next, in MessageIndex.template.php (must be done for each theme folder that has this file), find
<span id="msg_' . $topic['first_post']['id'] . '">
Change it to
<span id="msg_' . $topic['first_post']['id'] . '" title="', $topic['first_post']['preview'], '">
Congrats, you now have this enabled for your SMF 2.0.1 / 2.0.2 forums!