OK, after a bit of digging around it seems that get_comments() does not have any hooks at all and just grabs the raw comments. However, a quick fix is that you can manually pass any content through the wpcf_filter() function with the raw comment text (so long as the WP Content Filter Plugin is active). :)
I have tested this modified code and it works on my local install. It should give you what you need:
<div id="lastcomment">
<?php
$args = array(
'status' => 'approve',
'number' => '1',
'post_id' => $post->ID(), // use post_id, not post_ID
);
$comments = get_comments($args);
foreach($comments as $comm) :
$comment_author = $comm->comment_author;
$comment_content = $comm->comment_content;
if (function_exists('wpcf_filter')) { // make sure the WP Content Filter Plugin is still active, and available!
$comment_author = wpcf_filter($comment_author);
$comment_content = wpcf_filter($comment_content);
}
echo('<span style="font-weight:bold;">' . $comment_author . '</span> said: ' . $comment_content);
endforeach;
?>
</div>