Try:

<?php
$comments = get_comments('status=approve&number=5');
  foreach($comments as $comm) :?>
 <div class="index_row_entry <?=($i%2)?"color1":"color2";$i++;?>">

    Post: <strong><?php echo get_the_title($comm->comment_post_ID);?></strong><br />
    Who: <?php echo($comm->comment_author);?><br />
    What was said: <?php echo($comm->comment_content);?><br />
    Date: <?php echo($comm->comment_date);?><br />
    </div>
  <?php endforeach;?>

That uses the comment_post_ID field from the $comm array and passes it to the get_the_title function. For a fancier look, including a link to the comment permalink, use:

<a href="<?php echo get_permalink($comm->comment_post_ID);?>#comment-<?php comment_ID() ?>"><?php echo get_the_title($comm->comment_post_ID);?></a>

Hope that helps.

there was a typo in the code by @mtw28 which might have stopped the code from working fully.

comment_post_ID id indeed part of the wpdb comments table:
http://codex.wordpress.org/Database_Description#Table:_wp_comments

the fixed code below works for me:

<?php
$comments = get_comments('status=approve&number=5');
  foreach($comments as $comm) :?>
 <div class="index_row_entry">

    Comment on: <strong><a href="<?php echo get_permalink($comm->comment_post_ID); ?>"><?php echo get_the_title($comm->comment_post_ID); ?></a></strong><br />
    <?php echo($comm->comment_author);?>: <?php echo($comm->comment_content);?><br />
    <?php echo($comm->comment_date);?><br />
    </div>
  <?php endforeach;?>

ps:
if you want to know all elements of the get_comments() result, try and use:

<?php $comments = get_comments('status=approve&number=5');
print_r($comments); ?>